I just rewatched "Meanwhile" today, and I have to say, for a chick who clearly never wears a bra, Leela's breasts really aged well. Not much sagging. How unfair.
Not quite, young Padawan.
In the first Futurama comic, Bender wants to prove that the X-Ray googles he purchased are working, by telling Leela that she wears a bra. To which she replies that she alway does (indirectly hinting that those proof does not count, because she considers herself wearing a bra something obvious that anyone could guess).
Also, in 2-D Blacktop, Farnsworth said "...when you upgrade your bazooms". So, Leela got at least one boob job. (Regarding the possibilities of genetical engineering, it might possibly have been done without implants. I am pretty sure that someone offering cosmetic surgery - but leaving the women still the right to say "Yes, they are real"- would earn big time).