Thanks — this is worth fixing, and you are right that it is not consistent.

The extension was taking the first entry Instagram sends and assuming it is the largest. That assumption turns out to be wrong: the list is ordered by encoding profile, not by resolution, so on some posts the first entry is not the biggest one. The next version picks by actual pixel size instead.

One thing to set expectations: Instagram itself serves these images at up to about 1080px for most posts, so there is a ceiling we cannot go above — what the extension can do is always give you the largest thing Instagram actually has for that post, which is what the fix is about.