It makes sense to me. Perhaps the officers handling accidents or the volunteers doing their stuff can get by on mid size cars. The regular patrol work would stay with full size cars.
Actually, they didn't use ANY full size sedans...one of the old "Cops" episodes airing on TruTV showed an episode in Pittsburgh in 1991, and showed mainly Pontiac Bonnevilles and Olds Delta 88s of the era. I found it kinda weird that Pittsburgh didn't use any Chevrolet Caprice (or maybe they did and I haven't found them yet).