There is some connection between overall body weight and fertility but that's about it. I had some trouble conceiving until I gained about 7 lbs (years ago). Didn't make much difference in my teeny little breast size but my hormone levels seem to perk up a bit. I've had 3 kids. By the way, breast size doesn't matter when it comes to nursing either. I nursed all 3 of my kids at least a year each with my little doorbells. All the milk glands worked just fine.
Size has nothing to do with fertility breast are just made out of fatty tissue that's it. She would be just fine to have kids.