A slider to change breast size. Works well and fairly precisely on the medium to large end of the size spectrum. Above 1.7 ish you’ll start getting ridiculously big, extra large or monstrously huge but not necessarily in a linear fashion. It is less linear in the smaller sizes until you reach a point (@ about -1.2) were it gives truly very small breasts but CAUTION the checkpoints I tested it on equated small breasts with youth or infirm/starving.

My theory is illustrious or at least the model I favor (Prefect Illustrious) have built in biases toward big breasts so the fight between the slider and the model leads to an averaging or at least dampening effect on the low end until a threshold is reaching where the model stops fighting and puts the subject into a category other that healthy adult woman.

I suggest anchoring it in unambiguous adult promoting such as “1girl, adult woman, 30 years old…” though the age is arbitrary it just seemed firmly in the adult woman range