I really don't give a single shit about how she looks. There are women with large breasts and women with small breasts, so either is "realistic" in the sense that they depict a person. In a world where people swing 20 foot swords or a cat riding a puppet punches out a dragon, that's good enough. I just hope they don't make it like a teen romance (even though that is in some of the flashbacks) and keep her as an ass-kicking mother type. That is her character and important to all the relational dynamics between everyone.
My main concern is the fact the chibi style and overhead whole-scene camera allowed some things to be done and not taken too seriously. I mean some of the scenarios in the game are really fucked up if you think about them, but you don't have to because it's so unrealistic. A fully realized and realistic world wouldn't convey the same thing with Cloud dressing up as a girl to infiltrate a human trafficking bordello to save his friends who are about to get raped by a slumlord, or genetic experimentation on and slaughter of innocents with a funny/goofy president at the top managing it.