The A10 uses a 30mm and is designed to account for the monstrous recoil by off setting it. So larger than a Vulcan. HELIX frames are much larger and more advanced than a modern day fighter, and it'd be safe to say that as the machine is upsized, so too are the weapons. There's also the consideration that technology has rapidly advanced too. A simple solution for the magazine capacity would be that the ammunition is cashless, and thus is easier to store and lighter in weight. The A10's 30mm cannon also fires like... 30% faster than what the 90mm is spinning at.

On top of all that, this setting is based on the Armored Core universe, and some of the ballistic weaponry in that series is straight up completely impractical and not feasible. In that instance, the rule of cool applies. :P