It's more like, other animals don't have breasts at all when they aren't lactating. No specific sources to suggest it's sexual selection, I heard it somewhere. I like watching videos about archaeology but it's not my field. Breasts don't really fossilize, so don't think we'll ever know for sure. Idk about nipple size, but yeah too small isn't great.

To clarify, I'm not saying specifically large breasts were selected for but that the fact humans have breasts at all suggests it's at least a secondary sex charactaristic (like beards) and I don't think it benefits fitness in other ways

edit: Probably saying "big tiddies" was not the right way to put that