Ah yes, that's what I understood, apologies. Well then that's pretty much the most you can do for now. The sprite changes every ~15kg, and in the first stages the changes are a bit subtle.
I'll implement other ways to fatten up NPCs. Once I release the next update, I'll start working on special events, where you can invite a NPC over and eventually do a stuffing / force-feeding session. This'll make the process a bit less of a grindfest.