https://github.com/openjdk/tsan/pull/17, there are more than 5000 commits in this pr and mlbridge bot was trying to generate the webrev. The generation of webrev was successful but the webrev file was too big and exceeded GitHub's file size limit.
Here is the error we get from GitHub:
remote: error: Trace: b35b6d85b1af6c8bcd716e6c56bac43a28f74ef046096faf2013bff8ba230177
remote: error: See https://gh.io/lfs for more information.
remote: error: File tsan/17/01/comparison.json is 112.74 MB; this exceeds GitHub's file size limit of 100.00 MB
remote: error: GH001: Large files detected. You may want to try Git Large File Storage - https://git-lfs.github.com.

So as ErikJ suggested, if the diff of the pr is too large, we shouldn't generate webrev for it.