Why is there no reason? For the pokemon that haven't had a person to measure them before, how do you know that isn't the only size they have?>>37100795
Actually, the original post only said it increases by size, and if you increase only by size, then your density will decrease. However, I know us to be talking about two different objects, and even for two of the same species of animal at different sizes, we cannot assume the same density. They will have different masses.