i haven't gotten endings yet (idk if there are endings) but she got fat when i spammed some food items..not very good strategy, that was by luck
I got burger and fries when I inserted 200 hm
I presume fast eating, keeping her hunger bar high and feeding semi normal foods might give that
edit: struggling to get the fat transform again despite regularly eating the fast food items and overdosing food
my guess is you just need to eat many food items (while looking regular) in very short time but i don't have enough food items to prove that rn