Somebody tell me, bigger rotor produce higher temperature due to more watts to spin bigger rotor. Is this same as higher Kv ?
A bigger rotor does not necessarily produce higher temperature. This is a function of thermal design. A bigger rotor could run cooler in some cases. The heat in a motor has a lot to do with whether it is powering a relatively heavy load (making more torque and drawing more current than it is designed to handle).
Kv is the air gap voltage constant in RPM/V. The motor coils absorb current and produce torque, but under commutation, other coils move through the air gap and generate reverse voltage. The maximum theoretical speed is:
nMax = Kv*Vs
where Vs is the source voltage. In practice Kv is given with +/- 15% or less precision and there's friction and windage, so nMax is a fairly rough estimate.
In a frictionless environment the motor cannot exceed nMax because Kv is a negative feedback source inside the air gap, limiting reverse voltage to no more than the battery voltage Vs at the steady state top speed.
2ndly, bigger rotor means more magnetic strength ?
The type of magnetic materials determine strength, not the size of the rotor. A smaller diameter rotor can contain stonger magnets.
Larger overall machines of course make more power, generally with a lower value of Kv and greater torque, but the source of extra power is via absorbing more electrical power and distributing the magnetic flux linkage over greater surface area in the air gap. The magnets are not necessarily stronger, the heat not necessarily greater, the rotor size does not necessarily increase power, and Kv is a property of the magnet strength and number of turns.