I'm surprised that JWST isn't actually all that much bulkier than Hubble, despite the mirror(s) being so much larger. In retrospect I guess that makes sense, since much of Hubble's bulk is the long tube enclosing the mirror, which JWST lacks. Also, the size of things that can be reasonably launched into space is limited by the launch vehicles themselves. (Maybe NASA would have built it bigger if they knew exactly the launch vehicle specs that would be available at the end of 2021, or they might have waited for Starship, but obviously that couldn't have been predicted when work on JWST began.)