Reptile basic hides

[The Serpent Merchant]

Maybe my BP is an odd duck, but I have a ~4 month old BP who last weighed in at 256g.
He wears the small size like a turtle shell at this point. He spills out of the sides, lifting it off the ground on 3 if not 4 sides every time he attempts to go inside.
I currently have 2 mediums and 1 small in his tub. He uses all three, but generally seems to prefer the mediums.

How a 300g+ snake fits in there is unclear to me.

They don't. They turtle them but they still work as hides. The mediums are just so much bigger that I wait as long as I can before upgrading.