Why Does Generator Rotor Speed Decrease with Higher Ratings?

[DUET]

Today I observe some generators of different rating which have designed by a company called X. I notice that the speed of the rotor reduce as the rating of the generator increased.

Could someone please explain it to me why?

 

[Baluncore]

You have not specified DC generator or AC alternator.

More power requires thicker copper and more magnetic material. That makes it a bigger diameter rotor. More circumference uses more poles of optimum aspect ratio, so less speed is needed to generate the same frequency AC power.