i don't think physical size is taken into account in terms of gameplay mechanics, and is purely asthetic.

what matters is whether you gave your giant elephantine creatures buzzsaws, pincers, teeth, etc.
at least that's what appears to be the case.
to be honest, i wouldn't be surprised if the game allows something like a willosaur to drag the carcass of an epic monster around, since, from what i've seen/read mass/weight is not considered in the game mechanics