One thing that actually just occurred to me but, isn't Tifa like a master fighter? I mean she's a martial artist, apparently powerful enough to PUNCH robots in the face, and they break, not her. So, I would assume she trains, and is as strong as your average MMA fighter, if not exponentially more so. And...none of them really have massive tits. It's kind of antithetical to the whole "I train non-stop to hone my body into a living weapon" type of diet/exercise that they would go through. Boobs are one of the first things to go naturally. So, it's frankly more realistic, not just on the concept of "most women have smaller boobs" (because my wife has massive ones, and she bitches about them daily), but from the standpoint of "Tifa is a powerful fighter" angle as well. Someone with her background isn't likely to have giant tits, at least not without some plastic surgery to compensate for the insane diet and exercise.
I mean women who have massive tits, and who regularly exercise, have to frequently wear like 2-3 sports bras, to keep the girls in check. Otherwise they flop around as they move, causing pain, and knocking them off balance. Again, my wife doesn't even exercise, has massive tits, and one sports bra is still not enough to actually give her the support she needs, while simultaneously keeping the girls in check and stationary.
So yeah, not even counting the "stop sexually objectifying game girls to massive tittied sex dolls", she makes more sense as a smaller chested woman, BECAUSE of what her character is and does.