Indeed they do all refer to the same thing, but you are right, they do all have different connotations.
Seno - Cleavage
The space between the breasts; The space between the chest and a woman's shirt. Seno
refers to this area, and you might possibly bring it up when a woman grabs her wallet from inside her shirt; or change, a cellphone, tampons, whatever... it's like a pocket.
Pecho - Chest
The area where the breasts are; the chest. If I am looking at a woman's chest, I am pretty much staring at her boobs. So use this one when you are referring to that general area.
Busto - Bust
Measurement. Plain and simple. It can also be used to describe a woman's boobs that are very big; so big that they stretch her shirt out.
Mama - Mammary
I've never heard anyone use this. Since I am not a native speaker though, I can only assume things. The use of motherly words is often used to express vulgarity, so this could be a very vulgar use for the word. Or, it is just short for mammary.
Chichi - Boobie
I have heard this used before, and it was at a party in Mexico. The word was spoken by an English girl so there's no telling where she was influenced to use that word, but I heard it. It was used in the same context as boobies
in English; to be cute.
Teta - Tit
In English, this word is usually thrown around misogynistically. I have heard and understand the word teta
to be equal to our tit
. It's sexual undertones make it an arousing word to use. And on the other hand, cows have tits too. Tits are where the milk comes from! :D
Each word is completely different from one another. Being a non-native speaker, I can't exactly narrow down regional usage for you, but I can point out their differences like I did.