Not true... Increasing a rotor diameter by as little as 1" can increase the effective radius significantly. This increases the lever arm on of the caliper around the hub and the output torque of the brake can be increased by 10% or more. The increased rotational inertia of the rotor (due to the larger diameter and slight increase in weight), although it does make it very slightly harder to stop the rotor, is insignificant compared to the overall inertia of the system (entire vehicle), and the increase lever arm MORE than makes up for the increased inertia.
Most OEM's try and keep rotor size/weight down for: BIG #1) COST, #2) Handling Considerations, almost meaningless in a truck #3) Wheel size packaging.