Edit: Actually I reckon they deliberately oversized the container a bit so it's easier to pack the cash in. You don't want to build it too small! (Relative budget notwithstanding). Another design constraint it has to be a cube, and has to fit nicely to the dimensions of the banknotes on the front face (aspect ratio and size) without having a big gap on one side.

Well, no, it's kind of like ordering two burgers and getting three.