There's all the accusations that Tifa Lockheart has had breast implants. To those people who believe that: Are you people idiots?
First of all, what would be implanted? Hmm? PLASTIC? Uh-uh, no way, they would be too toxic. I mean, have you SEEN Midgar, the only place where a plastic surgeon _might_ be? Not the cleaniest place in the universe. That goes for most other types of implanting material.
Secondly, who would she trust to put them in? I didn't see a huge surge of doctors, as the Sephiroth Clone in Sector 5 showed. Most people would just buy a Materia to heal most ailments or get friends who had the Materia. Seeing a doctor would have been for pregnancies or mysterious ailments.
Thirdly, who would pay for them? As we saw with Barrett, they didn't have much money. And Tifa was a member of Avalanche and also good enough friends with Barrett to take care of Marlene. Loaning money wouldn't have been a problem if they had a lot. So, obviously, they didn't.
Fourthly, why bother? Cloud was obviously the only one who caught her eyes when she was young, so she would only get them if he asked her to. And he barely had the gall to ask her to the waterwell, let alone tell her he wanted her to have large boobs. Hell, he was too ashamed of the fact that he wasn't able to be in SOLDIER, he would have never had the nuts to tell her to get large breasts. Anytime afterward, he was imitating Zack, so unless Zack had told him about a large boob fetish, he wouldn't have even mentioned it.
And finally, unless there's some sort of Breast Enlargement Materia that comes free with every purchase of another Materia (which would have resulted in a lot of large breasted women on that world), it would have been too expensive to waste the money on.