There seem to be some misconceptions floating around on the web about whether large disc rotors are inherently less safe than smaller rotors. The concern is that a large rotor applies a larger torque than a small rotor and hence may shear off the brake calipers.
When you are locking the wheel with a large rotor, the maximum torque that you can apply will be exactly the same as when you are locking the wheel with a small rotor. Why? Because the maximum torque depends only on the frictional force between tire and ground, and on the radius of the wheel. That’s Newton’s 3rd law: the torque of ground-on-wheel = torque of wheel-on-ground. You can’t apply more breaking torque than when you are just about to skid your wheel. (At that point there is still static friction between the ground and the tire). If that’s safe to do with a small rotor, it’s exactly as safe to do with a large rotor.
To stop you and your bike from rolling forward, you apply a torque on the disc rotor opposite to the direction of rotation. To achieve the same breaking distance, a large rotor will require less pull on the brake lever than a small rotor. But, if your fingers are strong, there is no reason not to go with a small rotor, apart from saving some weight, if that makes you happy. However, heating is probably a major reason for choosing large rotors.
Here is another aspect: As said above: torque equals force multiplied by the length of the lever arm. The force that you apply to the disc rotor = (coefficient of friction of your brake pad) x (force between pad and rotor) x ( number of pads). You usually have 2 pads, but there is no reason that 1 wouldn’t be sufficient if things are designed properly. Also, the breaking force does not, to first order, depend on the area of contact between pad and rotor. A heavily drilled out rotor will stop you just as well as a solid disk – ignoring issues of clearing dirt and cooling.
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