ORIGINAL: HarryC
Craig, you are partly confusing the way a jet works with the way a fan/prop works. The turning of the turbine does not create the thrust, it is a windmill, the turbine creates a heck of a lot of drag! The exhaust velocity of a jet comes from the expansion of the air caused by heating, the exhaust velocity of a prop or fan comes from mechanically pulling and pushing the air backwards. The rpm of the jet turbine is designed to make the turbine blade speed reach an optimum so on a large diameter turbine you have to turn it at a much lower rpm than on a small diameter turbine.
Harry,
Of course you're right and after I had thought about it after my post realized the rpm is not the issue with a smaller turbine, rather the volume or mass flow as you more accurately described it. If the exhaust velocities are as high as suggested in several of these posts, then I would conclude that drag is a much more limiting/equalizing factor on top speed than the diameter of equal thrust producing turbines.
Thanks for the educated comments. It confirms what I suspected in a factual way.
Craig