Words mean things ... but not always what some people may think they mean.
For Sig, 7-rd capacity, and then 8-rd capacity, for their .45 ACP pistols has always been
normal.
The newest 10-rd double stack version would then be considered
high capacity, if you're comparing them to the older design/capacities. They seem to think of it that way, and it's their product to shill.
I remember when the Browning HP & S&W M59 were commonly called hi-cap guns. Why? Probably because they
were hi-cap guns compared to 1911's & M39's.
Not hard to understand. (This was in the 70's & 80's, BTW, before legislation ever dreamed about addressing magazine capacity.)
The canceled JCP military pistol program ('05-'06) listed a standard capacity for the then-proposed JCP .45 as no less than 10 rounds, and then stated high capacity magazines be available. Coincidentally enough, some commercial spin-off's of pistols designed at that time (with those program specs in mind) have given us 10-rd standard and 14-rd hi-cap mags. Imagine that.
http://www.cbd-net.com/index.php/search/show/893436 (but this older link didn't list specificity of mag capacity of either mag spec)
Now, if you want to think of the HGK USP 45 and Glock G21 as having "standard" capacity magazines because they were able to be built around mag bodies which held more than 10-rds, go right ahead.
Doesn't mean the rest of the handgun world is going to think of them as the established "bar" of what standard & hi-cap .45 mags ought to be, though.
As a long time owner & user of 1911's, beginning back when 7-rd was the only mag available, and then carrying either a 4566TSW (8-rd mag) or a 4513TSW (7-rd mag) as issued weapons ... I tend to think of my M&P 45, with its 10-rd standard mag, as "hi-cap".
But that's just me.