So, you are saying a 20mm tire would be fine for a tubular, but not for a clincher because of the thickness of the rim bed?
Plenty of people ride 20mm tubulars. General physics state that the size of the contact patch is dependant on the weight and tire pressure (not on size of the tire or construction of the tire - those affect shape, but not size).
Just wondering why a 20mm clincher woudld be different...