You could add a mini game (just a few clicks) like her having to carry heavy weights (wooden alcohol boxes) somewhere each day.
You can reward this with a few coins but much more important is that it could imply some nice hidden feature: over time her BIG JUICY ASS gaining further in size or loosing it over time if stopped practicing.
Remember fitness in San Andreas, it added so much charm to the game.
This would be a nice way of adding ass size choice, but without the classic Sims pick-at-start way.
The mini game can be just a few clicks but many 3Ds would have to be slightly edited.
Interesting developer question: can ASS SIZE be automated in this type of work. I'm sure you keep all the scenes saved so, would each ass have to be redone manually ?