Originally Posted by Sixty Fiver

Incorrect.

Your arguement is actually self defeating as if you state the the hub is spinning faster on a smaller wheel, then the smaller wheel is also moving at a higher rpm for a given speed than a larger wheel.

Comparing actual wheel sizes here...

A 406 mm wheel turns at 168 rpm at 10 miles per hour, a 622 turns at 123 rpm, at 30mph the 406 turns at 504 rpm and the 622 wheel turns at 368 rpm.

So if a bottle generator on a 700c wheel needs to have the bike rolling at 10 mph to bring it up to full power, the smaller wheel only has to be turning at 6.5 mph to spin the bottle generator at the same speed as this is where the rotational speeds of the wheels match.

Whether it is a hub generator or bottle dynamo, a smaller wheel does not have to turn as fast to deliver the same output as a larger wheel