You need access to the lab, the character there, Tiffany, will eventually give you the item you need to make Diana bigger.
As far as I know, that item doesn't work for and isn't used for her, the vial labeled WG only works on the owner of Shadowed Curios as far as I know. If you want to make Diana fatter, the only way I'm aware of is to just keep playing the game and gaining weight until the lab opens up as an option to travel to (I can't remember the exact weight that triggers its introduction, but I think it was somewhere past 200 pounds IIRC), the character you interact with at the lab will be the one to give you what you need to help Diana gain weight.