Did you realize the main size difference between a USP and PX4 is...
I just compared my H&K USP (full size) to my Beretta PX4 - I was pretty surprised! All the dimensions are essentially the same, width and height, with the USP being only about 3/8" longer. The thickness from the top of the slide (in front of the trigger guard) is very close to the same.There is less than one ounce difference in weight. The USP's girth around the grip is significantly larger although I didn't measure that.
The USP has always looked like a large gun to me and the PX4 more compact; but except for grip girth, they're essentially the same.
USP looks bigger than it is
I also have a full size USP, in .45 caliber, and have compared it to other guns I own. Although the USP "looks" big and has a reputation as a large gun, it is really about the same size as some other commonly used handguns. For example, I have a Sig P226 in 9mm, and the USP is very close in size to the Sig. They will fit in the same holsters, and are the same length and height. The USP actually has a grip that is smaller in width than the P226, and weighs a couple of ounces less than the Sig.
I think there is something about the design and appearance of the USP that makes it look bigger than it is.