When my younger sister was about 3 years old, she would try to get inside my toy cars. Like 2 inch long Hotwheels cars. She simply didn't have the capacity for understanding spacial relations at all. She would cry and freak out that she couldn't fit inside the tiny car. This reminds me of that.
The more more small box is the best. You can clearly see his train of thought:
"This box is too small to be a body box, therefor it is clearly a head box."
I knew a fat cat who would only lay in boxes that were slightly too small for him. He seemed to enjoy the way his flab would push out over the lip of the box. A fluffy souffl�.