My summer fruit flies get bigger in the fall and eat the cat food. What are these bigger flies?

I have a lot of fruit around, and a lot of fruit flies in the summer, but I catch them easily in the traps. But when it starts to get cooler outside, they get bigger, like 4 times the size, darker, and they jump around spastically really fast, not like regular fruit flies at all -- and they don't go into the fruit traps -- they only eat the cat food. These bigger ones land on my skin and I don't like them at all -- what are they and how can I get rid of them? thanks!