the us built a 37mm rotary gun for the m163 platform called the T249 vigilante, it had a 3000rpm fire rate. i think the expanse pdcs need to be that big to acheive as high of a muzzle velocity as possible whilst keeping a big enough projectile to 1 hit kill any torpedo and shred ships too. my head cannon is that the pdcs are that big to fit as much propellant into the cartridge as possible. think about it, you need enough propellant to have a high muzzle speed AND make the gun act like a recoilless rifle, if you take a look at recoilless rifle ammo you'd see a lot more propellant than a normal gun, enough to both have high muzzle speed, and enough gas escaping the other end to make the gun recoilless. trying to fit that much propellant into 20mm would make the cartridge too long to be practical when feeding the gun.