Prop diameter has far more influence on thrust than RPM does.
Thrust goes up - or down - as the 4th power of the diameter but only the cube of the change in RPM.
In other words, if you go from say 4000 rpm to 8000 rpm, thats doubling the rpm you should get 2 x 2 x 2 = 8 times the thrust.
But if you double the diameter of the prop you will get 2 x 2 x 2 x 2 = 16 times the thrust.
Another more realistic example might be a 10% increase.
With rpm if you go up by 10% you get a thrust increase of 1.1 x 1.1 x 1.1 = 1.33 times as much thrust or a 33% increase in thrust for a 10% increase in rpm.
With diameter, if you do the same 10% increase - say going from a 10" to an 11" prop - you get 1.1 x 1.1 x 1.1 x 1.1 = 1.46 times the thrust.
Power usage goes up or down at the same rates.
At least, thats the theory