IMO it is appropriate to refer to the following as high capacity:
Magpul D-60: An AR15 platform drum mag that holds 60 rounds of 5.56/.223
Glock 9mm 33 round magazine
Magpul D50 (.308 variety of above)
You may also prefer to call them “extra capacity” or “extended” or something.
Not that there is anything wrong with “high capacity”, those are at least as protected by the 2A and logic and reason, from infringement, as more typical or standard capacities like 30 round 5.56 mags or 17 round pistol mags.
But to the point of the OP, it is just plain wrong/incorrect to call a 30 round 5.56 mag or 17 round pistol mag “high capacity”, 'cause those are standard and typical
I think high capacity is any amount that would hinder one from the ability to defend themselves. It is an individual thing.