If I remember correctly, the literal translation of "chest" is the polite way to refer to a woman's breasts - what you're refering to is implied, which is pretty common in Japanese. "Oppai" is a more explicit way, like "boobs" or "breasts", either of which might be appropriate translations based on context.
I don't agree that "oppai" is only used for large breasts. I suspect it's just that someone isn't as likely to call out a woman's smaller breasts. In IESP-647, two girls are talking/fondling, and the younger says (in my translation) "Unlike yours, my breasts are small, right?", and she definitely uses "oppai" to describe them.