three more ideas, use if you like
Plant girl who starts out the size of a potted plant, ends up growing into and around a large, warehouse type building, roots and flowering vines overtaking the structure
Robot/ automaton inventor girl who creates an artificial Digestion system for herself just because. starts out fully analytical about eating and stuff, but slowly gets very into it ("ive been using my excess power for freelance processing, and then using the money for food delivery. results appear exponential.")
last one is a bit out there, and pretty speciffic:
Normal human girl with a monsterous alter ego (split personality, alter ego looks like a true monster but mostly just feels bad for being a burden on the human.) but, after you feed them for several stages, something unique happens. in a special scene, they undergo a sort of divide/mutation where they end up as two split entities, both only semi-monsterous (the cute kind), and from then on they act like a pair of sisters, being fed together