Generally speaking, for all but the smallest of changes you should never only review the diff, but pop open the actual file on that branch (GitHub makes this fairly easy, I can't speak to other tooling) and consider the change in the context of the surrounding code. Does it contradict comments? Needlessly duplicate code from elsewhere in the file instead of creating a single function to be used in both places? Shadow variables confusingly? Add yet another 5 lines to a 100 line function instead of leaving the code better than it found it?

You can't really judge any of this if you're just eyeballing a diff. Diffs are just the starting point for a review, and while tooling could make this easier, the full file is always just a git checkout away.