Haha, I actually searched really hard to try to figure out the right size and he still had to grow into it. Spatial things are just not my strength. I really expected him to grow out of it by now, but based on my amazon orders, nope, it's a medium, yes, it still fits. Reptile basics says ball pythons up to 700+g for mediums but nope, he can fit his butt in there just fine. It's certainly worth offering him the medium if you have it, but I doubt yellow's too upset about it. I didn't move mine up early because he likes squishing into it anyway. His first real size upgrade when he was a baby was after he couldn't fit in after a meal. I have a great picture of him with his butt in one hide and his face under another. I go with buy size upgrade early-ish, only swap in once they're wearing it as a hat and look ridiculous personally.