Well, the Lavos Humanoid is only a bit larger than a man... Perhaps a bit taller and wider, but nothing too big. If that's the case, Lavos Spawns would have Humanoids, too, but about the size of a human baby. Therefore, the Shells would be large enough to host them. However, due to their small size, they must be fragile (Like human babies, yes?) and if enough pressure is used on the shell they would be easily crushed. The Humanoid Lavos has about the same vitality (As in HP) as the mouth, so it's safe to say that Humanoids are only there to protect the already small core. If the Lavoid is underdeveloped, than heck, the Core could be as small as a cell in the early stages. Perhaps the Core comes out of the Humanoid Body as a collection of tissues (The Humanoid does send out the two Bits).