With all respect, you are wrong. The mag capacity is the same.
Check out the HK website.
You are wrong, actually.
There are 12 round mags for the USP 45 fullsize.
The HK45 has a 10 round magazine.
Do they make 10 round mags for the USP? Yes - for states with bans, and also during the AWB. But, he is right and you are incorrect.
See here:
http://www.midwayusa.com/viewproduct/?productnumber=869948