Steve,
I use a DT 'Spoke Calc' which requires manual measurement of rims & hubs, and I can confirm that indeed, tubulars are a larger diameter than 700c clinchers. In fact, tubulars are closer to 27" clincher than they are to 700c clincher.
For example: The largest tubular rim diameter I have in my data base is a Campy Lambda with a DT chart reading of 617. For comparison, a Super Champion has a reading of 612, which is the same as a contemporary Mavic SUP Tubular.
For the 700c clinchers, some readings are: Campy Moskva - 599; Mavic Open Pro - 602; Mavic Reflex - 604; Sun MA 14E - 601; Wolber Alpine - 612;
27" clinchers: Araya - 621; Weinmann - 625;
I have many more yet to be measured.........
Hope this helps...
Chuck Brooks
Malta, NY