My biggest boy (over 550 grams) is the same way with his RBI medium hide. He shoves his whole body inside and occasionally just has his head sticking out of the entrance. I offered him an exo-terra cave hide and he would not go in it. Same with the large RBI large hide, did not want to have anything to do with it. But he LOVES his medium hide.
But come to think about it, BP's do tend to like it slightly tighter for their hiding spot as it helps them feel secure.