If you look at those few pages, all of the sizes seem off. If we assume that Bobbin and his cronies are normal-sized, why is the three-eyed girl so tiny? And if we use Bobbin as a comparison, Big Mom is definitely bigger than Whitebeard, but not giant-sized.
Can't we assume that Bobbin and his homies are slightly above human sized (around Frankys size) and the 3 eyed girl is around Nami size but far away, as in a matter of perspective?
Because if that isn't the answer and Bobbin is normal sized then the girl has to be some kind of dwarf with a custom made chair to much her proportions?
I think the theory A is more believable, but then again I wouldn't be too surprised if she was just tiny, the One Piece has a lot of abnormally big people so it could be a great opportunity to add some abnormally small people aswell.
But now assuming that it is Theory A then Big Mam herself should be like Whitebeard when she is laying down and like a fat, just-under-giant height when she is standing (assuming she can stand).