You must have noticed by now that most things about sprint racing and mini sprint racing not to mention macro sprint racing as well are unique. The same goes for the tires and when you look closely at a complete sprint car you will notice that one of the rear tires are smaller than the other. You are not quite wrong there, yet in a way you are, they are the same size although the reason behind the difference us the stagger, which I will explain to you now.
A mini sprint car is not allowed to use larger than a 13-inch wheel and this is no problem as the way you fit and set the tires makes a big difference to the outcome of a race. Something else about the tires that is different is the tire pressure as a mini sprint car tires runs on low tire pressure for the rear tires with the two tires pressure two PSI different with the left rear as low as 4PSI. The reason for this is increased grip when you race on a slick or dry track and obviously when on hooky or very tacky track surfaces you use higher pressures.
For the same reason that you use low pressure must you always use bead locks on your wheels especially the rear tires must be run on bead locks. A bead lock is an essential ring that you bolt to the wheels outside tat locks the tires to place or they could peel from your wheels. Coming back to stagger it is easier to explain that the differential which allows rear wheels to rotate at different speeds during cornering.
For example when you turn right the left wheel turns faster than the right wheel and vice versa, and if not you cannot turn very well. With a mini sprint the rear axle is a one piece that connect the two wheels thus no differentials. In other words when you use two tires of the same size you will not be able to do great cornering.
This is how we do staggering depending on the track and its corners just before a race after you assessed the track conditions, you quickly remove bead locks and fit different circumference tires in order to achieve the desired amount of stagger.