Set CharacterName bowels/stomach/womb_capasity x
Examples:
Set Player stomach_capacity 9999
Set Yumi bowels_capacity 9999
Number can be anything you want, the visual size of bellies however caps at some point and will not get bigger with more capacity. You can modify pretty much any value in the game with the set comand (desires - set desire_inflate; contents of bellies - set stomach_water etc)