They're just size differences that affect stats, according to them:
>These two Pokémon also have got a unique form difference. However, unlike other form changes, these forms cannot be changed after capture. When you encounter the Pumpkaboo in the wilrd, it will be one of four possible sizes: Small, Average, Large and Super Size, with Super Size being the rarest. The difference is actually in their stats. While as a Pumpkaboo, its Hit Points go up, the larger it is, but the Speed decreases wheres when it has been traded to evolve into a Gourgeist, its Attack stat also starts getting larger for the larger forms while Speed decreases.
