I have a head canon that the ZPD's bias toward larger mammals has lead to Little Rodentia being grossly under served by law enforcement (after all, even Judy could kill several residents with one false step), leading to a rise in organized crime in that neighborhood. They're the only source of day to day protection those who live and own businesses in Little Rodentia have, so the either pay the mafia for it or be on their own in, essentially, the urban wilderness. Hence why Zootopia's scariest mob boss is a shrew!