JMR pretty well sums it up.
"30 caliber" is the diameter of the BULLET, not the CARTRIDGE. It's what comes out of the barrel.
These BULLETS come in different weights and shapes, depending on what is shooting it.
For example 50 caliber could mean a huge 50BMG used in a large machine gun, or it could be a bullet fired from a Desert Eagle handgun.
Very simplified explanation but you get the idea.