The real question is, WHY are men attracted to large-chested women? Most mammal species don't have large, visible mammary glands (with the exception of D&D races, where even the reptiles and insects seem to have them).
I've read theories that it is because when humans began walking upright, they needed new visual cues to health and fertility. That "health and fertility" are what guys are really looking for in a mate, but try complementing a woman on it and see what happens.
"My dear, you are looking extremely healthy and fertile today!"
I suspect it wouldn't work too well.
As for the barmaids, previous posters have suggested it is all about the money. I doubt medieval barmaids made much in the way of tips. They might make money at prostitution, but if they were any good at that, then why bother slogging mugs of ale to every smelly barbarian who wanders into town? I would guess the tavern owners have figured out that people buy more stuff if their happy, and men get happy looking at buxom wenches.
Corsets do indeed have a rather uplifting effect on the spirits, so to speak. They also fall into the category of torture devices evolved to make women more appealing to men, along with lead-based cosmetics, heavy wigs, chastity belts, high heels and pantyhose.
As for what the flat-chested women are doing, by traditional fantasy standards, they should all be out pretending to be men so that they can adventure/fight/sail/risk death and injury with the boys. That, of course, is only for the poor. Wealthy women are undoubtedly investing in amulets of disguise and such, which would also, in theory, eliminate the need for half-a-dozen maids to help them get dressed...
