Stafford has always had that "baby fat" face, which always made him look bigger to me. I was actually surprised to see his actual size (weight). I would have guessed about 10-15 lbs heavier based on his face alone.
Keep in mind that Austin still has two years to grow too, & I'm not calling them identical twins, but they aren't entirely dissimilar either. Both are somewhat traditionally undersized QBs from a height standpoint.
As for AK's actual height, I'm not sure. I'm just going off of his recruiting profile. Perhaps it is inflated, but I can't just assume as much and then assume that Stafford's is correct.