If people want a specific body type then they are free to ignore the toggle for this - most of them default to off - and those that want to option to do so have access to it.
I was thinking more along the lines of it grows as you use it, like a muscle and the fact the character comes out of a tube / tank in some sort of lab means the passive growth could be dismissed as lasting effects of a science experiment ... The character default name is lust doll so entirely possible that some of the chemicals used on the character may have lasting effects (human growth hormone or whatever the in-game equivalent is as an example).
As a matter of fact breast sizes can naturally change in adults - one of the major factors in that is the presence of estrogen and the female body produces more during puberty (where you are concerned about a mistaken age of the character if my guess is right) but are still produced in small amounts afterwards so breast growth in an older adult woman is plausible even without game lore reasons.
For men its less clear cut - most of the growth happens around puberty but in some cases it can continue to grow for into the 20s - an with the plausible reasoning of chemicals in the tube / tank having lasting effects and the rate of growth is slower then what might be common in children may make it more acceptable.
So how about in addition to a toggle have a note found somewhere in the unlocked early parts of the lab (the box with starting clothing might be the easiest) mentioning the growth chemicals used on the test tube subjects subjects (there is more then one tube) along with mentioning the age of the characters and a few other related details ?
Something like the following:
Subject: Code name 'Lust Doll'
Age: 20
Gender: [Auto fill gender here]
The application of the experimental growth chemical G-172 has produced exceptional results - character rate of growth is ahead of schedule and will to be ready for deployment at ...
(Rest of note is illegible)