Rotors:  Rotors come in various designs, but basically there is a vented rotors and solid rotors.  Rotors can be a one piece design with the hat or hub incorporated in the casting or the two piece design where the rotor is separate from the hat or hub.  In this case the bigger the rotor the better.  The bigger the diameter means it takes that much longer before the pad is in the same area during the rotation of the rotor.  This size also gives a mechanical leverage "advantage" when you increase the diameter of the rotor with the same calipers and master cylinder.

A good example of upgrading would be if you have a Mustang/Pinto 9" rotor and upgrade to a 11" rotor.  Not only does it allow more surface for cooling, it give a significant mechanical advantage.  This results in less pressure and brake force by the caliper to stop your vehicle.

On vented rotors the fins should be far enough apart to allow air to flow between the fins, but close enough together so it allows enough support for the rotor walls.  There can be as much as six tons of force being applied during braking and you do not want any deflection.  Some vented rotors have curved fins to allow better flow of air and maximizing the transfer of heat.   Don't forget these rotors can get up to 1,200 degrees F. so anything you can do to assist the transfer of heat is a plus.  If you have ever watched a NASCAR short track race with the camera on the rotors, you will know how hot they get.  

Solid rotors should never be used on a car weighing over 2,500 pounds.  These were designed for light duty and never used on a vehicle where heavy braking is needed.  These rotors serve a specific need and work very good under limited conditions.

Your better designed brake systems will have the rotor separate from the hat or hub.  This allows the rotor to have a uniform temperature across the rotor (remember the NASCAR rotor?).  By having this uniformity it minimizes the warping and cracking.  Wilwood for an example uses an aluminum hub in the front which the rotors bolt to. This allows the hot rotor to cool at the same rate throughout the rotor, because it is made of different material and it is a separate part.  The aluminum hub is also designed to displace heat and keep it away from the bearings (remember the modulus of elasticity number is 75 percent that of cast iron, meaning it will displace heat at a faster rate).  Having a two piece design also prevents the storage of this heat compared to a one piece cast iron rotor.  Calipers also benefit by having less heat transferred to them and it assists to keep the brake fluid under the boiling temperature.

Factories and cheaper brake kits use the one piece design, incorporating the hub/hat with the rotor, this was done strictly for cost.  Notice that the hub and rotor is cast as one piece.  This does not allow for the uniform distribution of heat and it is highly prone to warping and cracking due to the differences in temperature between the rotor and hub area.  The one piece cast iron rotor will also retain heat longer, thereby transferring excessive heat to the calipers and brake fluid.  The only advantage to the one piece design is initial cost.   Did you really save any money?  You would never see a one (1) piece rotor on a race car.

It would take pages to explain the relationship the rotor diameter has on the braking force.  To make things as simple as I can it basically is the relation of the old rotor size vs the new rotor size. 

Example: No change in tire size, going from a 11” rotor to 13” rotor.  The effective radius of the 11” with a 2” pad would be 9” and the 13” rotor with the same pad width would be 11”.  Or 11 ¸ 9 = 1.22 or 22% more efficient.

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