You don't need big tubular tires. First off, a tubular has more 'useful' air volume than a clincher/tubeless of the same nominal size. On a clincher rim, the air between the brake tracks does not serve any purpose: the purpose of an inflated tube/tire being to cushion the rim against the road surface. In a tubular tire, almost all of the air volume is useful.
Second, a tubular almost impossible to pinch flat. I've never done it, and I've hit filling-rattling objects that flattened the rim. Look at the tubular rim profile: a smooth circular section that cradles the tire. It lacks the pinch-flat causing sharp 'hooks' required to hold a clincher tire on.
Because tubulars do not pinch flat, you can run them at much lower pressures. Here is the sorry chain of events behind big clincher tires:
- The hooks on the clincher rim cause pinch flats.
- If we want a comfortable ride, we have to lower the tire pressure.
- So we have to increase the tire volume to protect against pinch flats.
- To compensate for the larger tires, we need wider rims.
- Wider rims and bigger tires are heavy and have high rolling resistance.
Plus we're all getting fatter. Tubulars avoid all of the above.