If you're supposed to produce a twenty page paper and you've got 13 pages, you need more meat in the paper to fulfill expectations. Alternatively, if you're supposed to produce a three page summary and you've got seven pages, you're getting caught in the weeds somewhere and need to take a step back.

It's a bit easier to, at the start of the paper, make a judgement about what's expected in terms of depth when you're given a page count instead of a word count, because you know roughly how much information can fit on a page but perhaps not how many words that takes up.