It is a standardized system, the gage of the wire or AWG (American Wire Gage) does get larger as the wire gets smaller in diameter and as the wire gets larger in diameter the gage gets smaller. Somehow they came up with table but it has been a standard for as long as I know.
Here is a link to a partial of wires gages and sizes.
http://www.rbeelectronics.com/wtable.htm