The student bodies aren't all that comparable. Although they both have 45k students, USC has twice as many graduate students as UCLA and they even outnumber the undergrads.

But really, its pretty foolish to compare an entire school to another. A given program against another given program? Sure. There are fields that USC and UCLA each might be at the top of, and there are fields where they are going to be much worse. That's the real rub with these rankings, combining and binning aspects of schools that aren't related at all. The only thing that's worth looking at is the rigor or research output of a specific program.