Its all in the sprocket gearing. If you have the same gearing running to the wheel for two different sizes of wheels, the smaller wheel will go slower than the bigger wheel.

If you think about the wheel size, it really determines the total distance that the wheel travels for one revolution. That old formula 2*pi*r gives you the circumference of a circle from its radius measurement. A bigger wheel will have a larger circumference, and therefore will travel farther for each revolution. Because of this the larger wheel will travel faster, given the same RPM as a smaller wheel.

You can alter the gearing to allow a small wheel to travel faster, which is why those scooters can go so fast with a small wheel.

This cuts through having overdrive transmissions with 1:4.11 gears (equivilant to sprocket teeth) with oversized rims and undersized tires.

When one throws monster mud tires on trucks they usually change their axle ratios to keep the same amount of power... and rpm at a specific speed. And they usually overdo it for more power down low at the expense of revs up high.

That being said, if you have a friction drive, the tire size makes ZERO difference whatsoever on the speed. With friction drives, the speed is dependent on the friction roller RPM and diameter, and on nothing else.

When one throws monster mud tires on trucks they usually change their axle ratios to keep the same amount of power... and rpm at a specific speed. And they usually overdo it for more power down low at the expense of revs up high.

That being said, if you have a friction drive, the tire size makes ZERO difference whatsoever on the speed. With friction drives, the speed is dependent on the friction roller RPM and diameter, and on nothing else.

How can that be possible? A friction roller will have to travel a greater distance on a larger wheel to make a single revolution of the wheel than it would on a smaller wheel. This means that a larger wheel will turn more slowly with a friction drive but go farther on each revolution, conversely a smaller wheel will make more revolutions with same friction drive but travel less distance per revolutions.

with a friction drive, the roller rolling on the tire is moving the same speed as if it was rolling on the ground behind the bike. The roller travels the same speed & distance as the opposite side of the tire, except in the opposite direction. So, you could have a 100 foot tall tire being driven by a roller and it would travel the same speed as a 26 inch tall tire driven by the same roller.

Another way of seeing this. A friction roller turns two revolutions to go one foot.
If the drive wheel is 10 inch or 30 inch diameter,that same two revolutions will still make you go one foot. If that friction roller is on the ground without a drive wheel and it turns two revolutions it will also go one foot.
Your wheel is nothing more than an extension to the ground.