We've figured out optimal packing methods for any number of squares inside a big square. When a number is below and near a square number like 15, you just leave an empty box, but when it's far from the next square number, you'll be able to pack them more efficiently than just leaving empty squares around. Turns out this kind of stuff is hilariously hard to prove that it's the most efficient method.

Mathematics actually hates humanity, and it likes to remind us of it, sometimes. That's why.

This is the most efficient (known) packing of 17 unit squares inside a square. If you're asking why it's like that, that's above my math proficiency level.

https://en.wikipedia.org/wiki/Square_packing

See also: https://kingbird.myphotos.cc/packing/squares_in_squares.html