you'll get a thicc sprite if you follow these steps:
1. get fairydust from defeating fairies
2. spray fairydust in the fairy area and gift them something (i recommend milk cuz i always own a ton of it)
3. play a game with the fairy queen
4. you'll have about a 40% success rate depending on your sense stat, so you'll probably have to try it a couple of times
5. if you win the screen will turn white and then you just have to ask to be thicc!
(also as a side note in the current version (r28.0) there a bunch of loading issues with various underwear and thicc and/or pregnant stat, so watch out and save a lot! but they should be fixed in the next release :) )
hope i could help!