Wire bit sizes

[Col. Batguano]

Just an academic question;
I understand how English bit sizes came about---X fractions of an inch. Same thing with metric sizes. But the diameters of the various wire bit sizes doesn't immediately make itself obvious. My suspicion is that it's X inches of steel wire to make a pound (or some other unit of uniform weight), similar to bore diameter or gauge, but I'm not entirely sure.

 

[Phil Coffins]

And while we’re at it how did letter sizes come about!
Then we can get into the hard questions like “pants” are one item yet “hair” is all those things that used to be on my head.