As a colleague wrote earlier today, Hollywood does totally has a longstanding love affair with fantasy armor, namely boob armor for women. A well thought-out piece with examples of the modern comic book movie take on female armor, it also had examples of the less mammary-shaped breastplates in current popular culture.
I, like my colleague, have larger breasticles that need a certain level of support in my daily life. I do not run marathons like my colleague, but I do lead a rather active life that includes things like renaissance fairs, conventions (both in costume and as a photographer), and various other activities that make proper support imperative.
My stance on the boob armor issue is different, however, because I believe in the power of a molded cup of support. If you've ever spent more than five hours in a corset, maybe you know what I'm talking about. A properly fitted garment used to shape the body and change posture can have more positive impact than negative, as the items can be for more than just fashion. You forget you're wearing the piece, if it's shaped correctly.
I've worn a full suit of armor, in the same day I wore a fitted and boned dress. I did it as an experiment to see which was more comfortable, and the dress won. Not because of weight, oddly enough — because with all the accoutrement and proper undergarments, they almost weigh the same. It was because of the support the steel bones and compression gave my torso (chest included), I was less sore by the end of my time in the dress.
It's the same concept with fantasy armor, where a piece molded to the wearer would make for a better ease of movement — especially depending on fighting style. If you are a fighter who needs to be lighter on your feet, like a Wonder Woman type, the less weight you have to wear, the better. So form-fitting, high-grade armor would probably do better for you than, let's say, the full suit that someone like Brienne would wear. A non-convex breastplate can also compress the breasts and chest in a way that prevents the wearer from taking full breaths during times of exertion.
I also have this theory that someone like Red Sonja can get away with wearing so little armor because she doesn't get hit as often while fighting. It means she's good; so good that she doesn't have to protect her clavicles with a gorget (depending on who's drawing her, though, because sometimes she has that really awesome shoulder dealy), or her shins with greaves. You could even use the cape defense here; that someone who wears a cape when fighting can use it to trap the opponent's weapon, if they're very good at it. Otherwise, yes, you're gonna get hung up on a barstool in the local tavern once in a while.
From a costume standpoint, both have their merits, but one isn't necessarily better or worse than the other. Like any good garment, the armor should be practical yet stylish, fitted however the wearer wants it to be, and an outward showing of the person's internal character.
Here's hoping we'll more fantastic examples of both on strong female characters in the coming cinematic years.