well, the smaller wheel is used, because the frame can be even smaller to fit the rider. Mainly, top tube can be shortened without making the HTA too slack or STA too steep. The saddle needs to be (X) distance behind the BB for the rider. Then the reach from the saddle to the bars needs to be adjusted, but if the rider is too short, this can be like doing a superman stretch, which is not really ideal for anything but time trials.

putting smaller wheels on a bike designed for larger wheels will render rim brakes useless and the BB will sit lower. A lowered BB will cause the pedals to sit lower and that can result in pedal strike.