Hmm,
Looking at the MM, The damage for a Huge Falchion is 2d6. The same damage for the claw of a huge dragon.
So based on this I'm considering that the natural weapon of a beast has the same size category of it.
This makes the same size category of a weapon to be smaller than its equivalent for creatures, which makes sense, since a medium mace is smaller than a normal human.
Unarmed strikes are not the same as natural weapons, so I think this rule is OK for me. What do you all think?