It's mostly a matter of pressure and mechanics.
The greater the volume, the lower the pressure with a fixed amount of gas. If we assume that the bolt (in lock up) nominally seals the rear of the bore, and the bullet nominally seals the front as it passes the gas port,the gas pressure in a longer barrel, using identical rounds, will be lower than in a short barrel. This translates to a lower bolt thrust, reduced wear on the carrier key and gas rings, and a lighter recoil impulse. It will also slow the rate of fire comparatively. That's a fairly simplistic answer, but it is it in a nutshell. Early on, the CAR system did have difficulties, but over the years those have been resolved to make it a fine operating system, although the mid length, with it's extra two inches of bore and tube is a better system. in my opinion.