The web is full of middle boxes with crazy limitations. And OAuth2 is very good of triggering each one of them. They are also mostly unique, not under the control of the ends, and often transient, so they most often aren't even understood, the problem is just assumed unsolvable. That alone is a big limitation that stops people from using OAuth2.

That said, I have never seen a case where crypto data was the cause of the bloat. Its size is so small when compared to everything else that I'm not sure why anybody would even look at it. And indeed, the rationale I found on the site is about cryptographic agility... what is interesting because you will find plenty of people claiming that this is an anti-feature that will harm security much more than help.