Rotor sizes and gauss meter readings aren't absolute either. Different brands of gauss meters will produce different numbers, and you can't really compare them between brands, and they also may not measure the same between meters of the same brand. I'm not sure how well the manufacturers calibrate the meters, if at all. There are tolerances on everything electronic.
All that the rotor diameter really determines is the "air gap" to the stator. This distance is usually different between motor designs. In general, the larger the air gap, the higher the RPM/less torque the motor has. This is just like a weaker rotor usually will have less torque/more RPM. The trick is finding the sweet spot (right combination) of rotor and stator for a given motor. That combination may be different for 1s vs. 2s classes.
For example, a weaker, smaller diameter rotor might be great in 1/12 scale, but a stronger, larger rotor may be the ticket for a touring car (assuming the same model of motor).