And many parts of large (+1GWe) reactors have also been manufactured at off-site factories and then shipped on-site by barges in the past.
This suggests one should move to reactor concepts that don't need such large structures. The containment building size is dictated by the need to contain a certain volume of pressurized steam in an accident (and the requirement to contain the pressure of that steam dictates the mass of the building's structure). This is perhaps the strongest motivation for reactors cooled with molten salt.
Alternately, allow steam to escape in an accident, after filtering. Most of the radioactivity could be captured. But this violates current rules that require no release of radioactivity for 24 hours in an accident.