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.