One of the authors sent out the first file. Another asked for a correction.
The first author made the correction and sent the second file. The whole thing, from sending out file #1 to sending out file #2 took maybe 2 minutes. It's not like the first author did anything fancy -- one word was changed.
I opened both files briefly to make sure I was deleting the correct one, and saw that one file was 157 kb and one was 281 kb.
One single word was changed. I do not understand how pdfs decide how big they are going to be.