Short PRs are better. My coworkers throw several tickets into a single PR. But the shorter the PR, the easier it is for the code reviewer to follow the logic, and thus notice flaws.
Also, when reviewing a long PR we tend to get fatigued and are more likely to just gloss over the changes.
10 lines of code = 10 issues.— I Am Devloper (@iamdevloper) November 5, 2013
500 lines of code = "looks fine."