308 is the bigger round....
The reason 7.62x39 is so cheap is that it was MAS produced by the billions of rounds, on a budget in China 20+ years ago.
The cases are steel NOT the more expensive brass.
The jackets are steel with .001" copper plating Not the expensive solid copper jacket
The powder is a single base mas produced Chinese military grade powder. Not a double base proprietary quality controlled powder.
The rounds were thrown together as fast as possible in china, Not precision made in America.