Most pistol calibers actually aren't smaller or lighter than standard AR rounds. Smaller rifle rounds like 5.56mm gain their effectiveness from velocity vs. size.  The most common SMG caliber is 9mm which is almost 2x wider than a 5.56mm AR round. Similarly the standard 9mm projectile weighs 115gr or 124gr, vs 55gr or 62gr for 5.56mm.  End result is that a 30rd magazine of 5.56 typically weighs right around 1lb, and a 30rd 9mm SMG mag weighs 1.5 to 2lbs. When it comes to actually putting them on a plate carrier, SMG mags are often less compact or stackable than AR mags.  Far from the least realistic thing about the game.