The feline species is known to make the most efficient use of any given space.
This is particularly valid if said space is encompassed by cardboard walls. The current
proof example holds not only an average member of the feline species, but also a number of hardcover books which, taken together, take up more space and weight than the feline, here already hidden underneath a paper layer. The feline also manages to fold down on itself to a size less than half the width, breadth and length of its original size (also know as feline Origami). Most surprising to the human mind is without doubt the fact that the feline does this out if its own volition and seems to find it immensely comfortable.
BTW, did you know that Amazon now also sells cats? ;o)