If the hoppers are only leaving a slight bulge in her belly I'd say Zim is gonna want to stick with the weaned.
If the hopper lump dissapears with 24 hours it's not enough food.
It might just be taking Zim a little bit to get used to the new bigger size, but don't doubt corns expanding nature As long as the weaned isn't more than 1.5x the biggest part Zim'll be fine.