That's because autocannons are with rare exception, burst-fire weapons. AC/20's are also almost certainly lower velocity shells reliant on mass instead of velocity to do their damage, so instead of one big slam out the back you've got multiple, smaller ones. Gauss Rifles also fire fucking massive 300mm projectiles made of solid nickel-iron, so I don't think your average autocannon comes anywhere close to the sheer mass of shot that gets shoved down range, even when you factor in bursts being fired.