In thinking about the expressiveness of Spanish vs. English, I was wondering:
An exhaustive list of words in any language is troublesome to impossible to make because of dialectal differences, English not having a regulatory authority like Spanish does (and even with Spanish there are slang words that the RAE doesn't recognize), and so on.
The DRAE consists of 83,431 entries (source), while Webster's Third New International Dictionary Unabridged has about 470,000 entries (source). That's a little less than six times as many words, although it should be taken into consideration that there are many regional dialectal and slang words that the RAE is not recording.
As for vocabulary, SpanishDict claims that the vocabulary of the average Spanish speaker is 10,000 words, but simple conversation only requires 1,000. For English, a question on Amazon's Askville cites a study that claims a conservative estimate of the vocabulary of a college graduate is around 20,000 words, and an ESL company claims that 15,000 words should be enough to read 98% of texts and that 3,000 words would be enough to get by on at minimum.
So, while English might have many times more words than Spanish does (or at least the "official" version that the RAE records), the vocabulary for English is on the whole much larger. That doesn't necessarily mean that Spanish is less "expressive", though; while it certainly might be more verbose sometimes, people will always find a way to say what they want to say, and they'll coin a word if they don't have one to use. (In fact, English's judicious use of loanwords might be one reason its vocabulary is so massive.) And all languages are constantly changing as the people and culture changes too, so in a few years it could very well be that the information provided here is outdated.
I tend to be skeptical about claims of "language A has x times many more words than language B". Personally, I have been using Spanish and English literally every single day for the last 12 years and I still haven't encountered a situation in which I had to use English because Spanish didn't have a word for the concept I was trying to express (or vice versa). I can point to several friends and colleagues who would agree with this statement. This is not the kind of thing you'd expect if English really had so many more words than Spanish.
The fact is that looking at dictionary size is an extremely unrealiable method of estimating vocabulary size for any given language. Dictionary size, if anything, is only correlated with the amount of money and effort that publishers and lexicographers were able/willing to pour into the project. Additionally, some publishers make different decisions about what to include in a dictionary. For example, Merriam-Webster (online version) includes a few thousand geographical and biographical entries, but DRAE (also online version) does not.
An alternative is to use a web crawler to compile a huge corpus of the language from online texts (see, for example, the COW corpus project) and then have a lemmatizer go at it. This might give you a better estimate, but it is not perfect either. For one, your corpus size (and, given Zipf's law, the number of different words) is going to be limited by the internet presence of the language you are interested in.
All in all, I would very surprised if English and Spanish (or German, or French, or Chinese, or...) happened to have a significantly different number of words.
This is a somehow extended comment and complement rather than an answer (I've already upvoted Ashen's answer).
The number of total words in a dictionary or the number of total words for college or basic conversation is sometimes biased by the meaning of word. Let's take one example out of many: “typewriter” v/s «máquina de escribir». The English counterpart is a composed word, the union of the noun “type”, the verb “to write” and the suffix “-er” (something that writes using types), but becomes one word with its own dictionary entry. The Spanish counterpart is also composed by three lexemes (content unit in language, including words, roots and adfixes), just that each lexeme is its own word and there is no individual entry in the dictionary for the concept. A literal translation into English would be “machine for writing”. Same concept, same number of lexemes, yet one dictionary entry more in one language than in the other.
On the other hand, dictionary words are mostly roots: the non-declined form of a word. So in a dictionary you will find the word “dog” but not the word “dogs”, and the word “(to) walk” but not the words “walks”, “walked”, “walking”.
The number of word variants in Spanish is much higher than the number of word variants in English. A regular verb in Spanish has over 50 conjugation forms compared to four different forms in English. Number and gender declinations in adjectives and articles. Gender declination in nouns sometimes adding extra meanings. Pronominal verbs having a shifted meaning without adding a different word as dictionary entry, etc.
Aditionally English has more irregular plurals, that deserve a different dictionary entry than the singular counterpart, and even Spanish irregular verbs have many patterns that dictionary authors do not care to provide most irregular forms as dictionary entries.
So one difference in counting words is what do you define as a word (and how a language works its mapping between lexemes and words) and what you count as different words.
The other difference in dictionary length are dictionary authors. The Spanish language has a very centralized and usually conservative body that regulates what goes into the official dictionary of the Spanish Language: Diccionario de la lengua española by Real Academia de la Lengua (RAE). Independent dictionary authors and brands do not go too far from RAE's dictionary. While RAE was the central Language Academy, until very recent years RAE was mostly a Spanish institution (Spanish as from Spain) while the Language Academies in other Spanish speaking countries were subordinated to the Royal Academy. Very regional words from a certain region in Spani Spain would be more prone to get into the official dictionary (v.g. «altor»), than words with more extensive use in Latin America. Before the ubiquitousness of the Internet era, the process for RAE to admit a new term, either coined in Spanish, borrowed from other language, or slang in any region of the Spanish speaking world, was a slow process, milestoned by editions of the printed work, and centralized in Spain. The process has changed and currently many new words such as «tuit» (tweet) have been accepted.
Dictionary authors in English have usually been more liberal in accepting new words, as they have a more descriptive than prescriptive approach. They want the dictionary users to know what that obscure word found in that obscure text by a English speaking writer means, rather than to tell dictionary users which is the correct form and use of the word.
So, how you count words («máquina de escribir» v/s “type-writ-er”) and how you count distinct words («caminar, camino, caminasa, caminando, caminase, caminó, caminara, caminará, caminado, etc.» v/s “walk, walks, walked, walking”) will make a difference in the number of word needed for college level or casual conversation. An dictionary authorship and criteria will also add a difference in words counted as dictionary entries. All of this without affecting general expressiveness in either language.
The fewer number of words in Spanish, however, doesn't mean that it can't be just as expressive as English; sometimes it is more so. One feature that Spanish has when compared to English is a flexible word order. Thus the distinction that is made in English between "dark night" and "gloomy night" might be made in Spanish by saying noche oscura and oscura noche, respectively. Spanish also has two verbs that are the rough equivalent of the English "to be," and the choice of verb can change the meaning (as perceived by English speakers) of other words in the sentence. Thus estoy enferma ("I am sick") is not the same as soy enferma ("I am sickly"). Spanish also has verb forms, including a much-used subjunctive mood, that can provide nuances of meaning sometimes absent in English. Finally, Spanish speakers frequently use suffixes to provide shades of meaning.
http://spanish.about.com/od/spanishvocabulary/a/size_of_spanish.htm
Bueno es tarde para responder a esto. Estoy de acuerdo en general con el resto de las respuestas, comparar las palabras reconocidas por los diccionarios no tiene sentido. Dudo mucho que algún día tengamos una estimación certera del número de palabras en español. Para que la comparación fuera justa deberíamos considerar que hay aproxiamdamente 93000 palabras en el Diccionario de la Lengua Española, 70000 en el Diccionario de Americanismos y 150000 en el Diccionario Histórico. Lo cual sumaría más de 300000 palabras. Pero es que aún así quedan muchas palabras informales, regionales y términos formales especializados que no son reconocidos por la RAE; por lo tanto no es posible hacer una comparación que sea significativa. Les recomiendo este artículo, en donde se explica con mejor detalle lo que quiero decir.
