How can you gaining the other girls' weight? Why they are always 40kg after eating outside? And how to use plug and minipump to them?
Other girls can’t visually gain weight, this is a hard limitation due to the game being 2D. They will be able to gain weight (and butt/breasts expansion) in the 3D Unreal version.
To use plug or minipump just drag the item out of your backpack (remember how you unlocked the door with the key at the beginning of the game?) and then give the item to the girl.