purely as a thought experiment, I wonder if given enough time, the NYC stray cat population would observe a genetic drift towards larger body size and start preying on the abundant local rat population? I never lived in NYC, and don't know how big the rats are there, but the rats I've seen (mostly at uni. biology labs) they grow to about 25cm in length (about the length from tip of index finger to tip of thumb with hand stretched out to the fullest for someone with large hands, not counting the tail), which I think is entirely within what a normal-sized adult cat can tackle.

urban rodents is a rich food source begging as an ecological niche to be filled.