Why can't they make the iPhone and iPad 1 mm thicker and put a bigger battery in there?
Good design doesn't stem from thoughts like this.
When designing the Mac Mini, Steve Jobs and Jony Ive looked at lots of different size cases.
The case size they settled on was only a couple of millimeters too small to fit in the a desktop size HDD (3.5"), which mean't they would have to use more expensive smaller HDD (2.5").
Did they change the design because of this? Make it an extra mm bigger?
Of course not.
The size they chose felt right and looked right.