It's worth mentioning that 7.62 ammo weights about 3X more than .223. It's my understanding most US military units use .233 largely for this reason.

The whole concept of .223 is to put more propellent behind a smaller projectile. This gives the projectile more speed which makes it exceptionally lethal and accurate at very long distance. Unlike handguns that basically poke a hole, .223 generates a shockwave when it strikes at a multiple of the speed of sound.