When applying the front brakes the quad lurches forward, thereby making a new center of gravity (more forward that when quad is sitting). By having larger circumference front tires you counter act that force being applied to the front of the quad. In essence, redistributing the weight to somewhere behind the front wheels. This in turn makes it harder (theoretically) to do stoppies/endos and gives greater applicability for using harder braking. If you have small front tires, the potential to flip would be greater and you would definitely not be able to apply brakes with any dependable amount of stopping power.
I'm not a scientist or a mathematician. Just trying to apply what little knowledge of basic rules of kinetic energy I know.