You could say that when a creature gets bigger, its mass is of course greater, and the increased ability to move it gets when larger is exactly balanced by its increased mass.
Not perfect. Ehh.
I asked this question a while ago and was told to look in the "Illithiad." I don't know if it is still available somewhere.
I was told that the creature grabs you so its mouth is somewhere on the back of yer head. Its tentacles secrete a powerful acid that eats through your skin and skull...