I hate this. Might as well be a parasite. Something about the fact they're wrapped in the flesh of something else will never feel right for me.
If you're going to turn the on-model bodies into fan service ones I'd prefer either some type of food/drink they just can't seem to get enough of even though it's making their bodies develop and grow, or cursed object that radiates an aura making them change but they are hypnotized by it and won't throw it away.
In both cases even though they are TFing physically (maybe even mentally if they are developing a bimbo or other persona that is common for their fanart) they won't heed friend/family warnings about it. Once they fully transform into their fan service version they then seemingly find it better and try to force the thing that changed them onto others to make them grow into fanart versions too.
Might even have different levels of growth depending on what fan art they are becoming. Say Gaz getting bigger tits >>127965815
and being too hooked to stop, rather than just becoming a fan art version she goes full monster thicc shortstack >>127965881 >>127965823
because she just won't give up the thing changing her. Then keeps going till she's even getting into BBW art versions of herself