I’m about to sound like a total loser, but I’m in Graphic Design and I’ve taken typography classes.
In most typefaces the lower case letters are set at an X height. This means that the the line at where the lines in an upper case x cross is where the body height of the lower case letters will touch. This excludes ascenders and descenders, of course. Also with most typefaces the x will fit perfectly inside a square. People are also visually attracted to things that are somewhat symmetrical.
Fun fact: Upper case and lower case letters are named for actual cases. When typography was done by hand all the letters were in a shelving unit that was split into cases. The “upper case” letters were literally in the upper case. The “lower case” letters were placed lower to help with productivity and back problems.
It’s sad how much useless info I have in the old noodle.