Typically, big brake kits only upgrade the two front brakes. This is because the front brakes do 70%-80% of the braking work on most vehicles most of the time (vehicle weight distribution, wheelbase, and center of gravity position all determine exactly how much braking is done by the front brakes vs. the rear). By upgrading the size of the front rotors, larger brake pads and better performing calipers can be used to increase stopping power. And when the kit is properly designed, it will still compliment the factory rear brakes.
Of course, there are rear big brake kits available for some vehicles, but they tend to be high-performance vehicles and/or large pickup trucks that tow/haul oversized loads.