well, where is the code for the hitbox stored? is it in overrides and the numbers on the second column?
It's all in the overrides/blah/something.cfg files. The lines "mins" and "maxs" specify the opposite corners of the bounding box relative to the origin. crouchMaxs, deadMins and deadMaxs specify the same corners when you're crouching and/or dead.
So the dretch's bounding box is 30x30x30 pixels and the granger's is 40x40x40. (All wallwalkers MUST have symmetric bounding boxes !) As the granger model is made for a distance of 20 from ground to origin, but the dretch bbox makes this distance only 15 the granger model intersects the ground.
There are two ways to fix this:
a) increase the "dretch" size to 40x40x40
b) scale down the granger model to the size of the dretch. Fortunately there's the "modelScale" parameter for this, try modelScale 0.75.