You would need to increase the size of the sun shield, structures, mechanisms, etc. A scaling up would cascade into a massive amount of design changes. For all

You would need to increase the size of the sun shield, structures, mechanisms, etc. A scaling up would cascade into a massive amount of design changes. For all we know, if JWST had been allotted 5x the volume they might not have gone with the current design. If we wanted to naively scale it up and "patch" what needed patching we might end up with an incredibly heavy spacecraft that isn't as good as it could be.