FWIW, I'm a published
FWIW, I'm a published biologist (or was at one time anyway), though evolution is not my specialty.
The concept we're all playing with here is called "
sexual selection" -- it's the development and continued existence of traits that don't seem to have any particular direct relevance to survival (such as gigantic pendulous mammaries). But if those traits make you attractive to mates, damn yes, they will continue into the next generation. There are some fun examples in nature like huge tail feathers on some birds.
Yeah, not all cultures see breasts as sexually as we do now, but that doesn't mean they're not subject to sexual selection. I think it's kind of reasonable for mates to be attracted to large breasts since they definitely are correlated with body fat, and body fat is (to our primitive eyes) a sign of being well-fed and therefore a potentially good mate.
Unfortunately this, like practically all evolutionary arguments, is really hard to disprove (disproving being the essential thing in science) because doing so really nicely would take long term manipulative experiments involving human breast size. Not something that's going to happen, for better or worse.
My bet is that in the past breast sizes were typically a bit smaller, especially among the kind of people who had to run frequently. And when they did, they probably tied them up. Check out the tunic on this ancient greek athlete, she has ONE breast covered oh so stylishly...