In one of my campaigns, the Drow female are extremely larger than the males who are the same size as halflings. Why not? Spiders often have extreme sexual dimorphism and I thought it'd be fun to add that to the drow. (They're spider-centric in my campaign but not evil.)
Interesting - you don't typically get that extreme of dimorphism in larger species (humans are towards the extremes of dimorphism for macrofauna), but hey whatever works (and bonus points in my book for making them non-evil!)
Historically armors have
OFTEN accentuated chests. See
Roman or Greek chest armor.
While it is true that a piece of armor is more effective when it directs blows away from the vital areas, a piece of metal is still a piece of metal, and will protect you regardless of how it is shaped. If you want boob-armor in your fantasy rpg, go for it. There is no historical basis for saying it is wrong or unrealistic.
I'm sympathetic to your viewpoint but I think you're slightly overstating the case. Chest plates of that kind (i.e. not angled ones as you see later) have always been subject to a lot of embellishment, because they're large, relatively flat areas which people will be seeing - on the same page you have a fabulous Japanese chest plate suggesting slightly overweight droopy old-man physique, which I imagine everyone, including the owner, thought was hilarious. If we're talking a tech level where people are using flat-plate-type chest armour, I think yes, if there are female warriors, there will be some boob-plate, some even perhaps of a very graphic nature, out there (I hesitate to link the currently-fashionable boob-plate that is around right now, but if one googles "Boob plate fashion" perhaps in incognito mode so as to prevent your targeted ads going wild, you'll see the sort of thing we might see).
However, if we're talking a civilization which is making more serious plate armour, like we see in the 1500s and onwards, which is often the case in D&D, where you have angled plates and so on, we will not see boob plate. This is where the problem comes. If you have the classic 1400s+ plate breastplate, it is angled to make blows/shots careen off it. The level of embellishment also tends to drop significantly. So if you have dudes in those, and women in boob plate, that's pretty silly and somewhat sexist. And it is wrong and unrealistic. People don't want to die, and when they realize they can make angled chest armour, it's not just going to be men wearing it.
Basically it's down to how you portray plate armour in your campaign - if it's 1400s+, esp. late 1400s or later, you won't have boob plate on either gender. Before that? You may well have it on both.