I would love to see some arguments for this claim.
I suggest reviewing the thread more then one page.
1) the M134 Minigun is the lightest Gun systems found in nose turret of a dedicated attack. Those were used in Early versions of the AH1 Cobra series and replaced by 20mm cannon as although fine for close range. The Russian Hind D started with the YAK-B 12.7x108mm Gatling gun. once again in operation excellent against soft vehicles and infantry but once both choppers were re targeted specifically against tanks what happened?
AH1J super Cobra turreted M197 electric cannon is a three-barreled electric Gatling-type rotary cannon.
Mi24P fixed Gryazev-Shipunov GSh-30-2
Mi35M Gryazev-Shipunov GSh-23 (ГШ-23) is a twin-barreled 23 mm autocannon.
Why? Higher range better HE potential improved anti vehicle anti infantry capacity.
2) second reason the smaller caliber rounds are not likely used for the Z10 is capacity vs rate of fire vs weight. since smaller caliber rounds are lighter and smaller a attack chopper can carry more rounds but since they lack power to compensate one would want a higher rate of fire. The best answer would be a Gatling gun. the Rate of fire and rotation makes these guns highly stable and very accurate. so then look at the Z10. a single skinny very long barrel cannon, why would you use that for a 12.7? the weight of a QJZ 89 is only what 39 pounds. So? If you wanted to turret mount a system with such a caliber why not go whole hog a Yak B only weighs 99 pounds, as to a heavier version well I have no concrete data but the 20mm M197 comes in at 132 pounds so a Comparable weight of a 3 barrel Gatling in 14.5x114 would probably fit in between. And the Russian GSH23 Twin barrel 23mm cannon is 108 pounds. above that the Bushmaster 25mm cannon is 262 pounds with a length of over 8 feet. the gun on the Z10 is not that long and it has a very plain profile seemingly lacking a muzzle break which are common for higher caliber cannons.
The PLA uses no known 20mm cannons they move form the 14.5mm to the 23mm round.
okay so what about the bigger guns? 25mm cannons are rare on choppers but reasonable. 30mm are the norm they offer fire power form Attach choppers that can take on most enemy threats and compatibility with IFV's the maximum I gave as my argument is 40mm and even that is a stretch such a gun of that type likely be based on either the CT40CTSW or the Bushmaster IV but in either case the gun system pushing over 800 pounds. That would demand a really heavily belt turret system or mounting housing. any larger and the recoil and weight well beyond reality.