Gas millage

Conventional cars lose mpg in winter same as a Prius.

energy in gas affects every car the same.
Less in the winter mix.
Air density also is independent of car and every car affected the same.
These two alone account for well over 10% to 15% variations summer to winter.

On short trips every regular car runs rich during warm up lowering mpg like a Prius.

The only thing the Prius is affected by extra is the hybrid battery when driving short trips in city driving and the car can't warm up. This is a minor part of the mileage loss.

Now if you compare to a car with a big V8 engine that wastes lots of fuel all the time, you will see less change in winter. But Prius vs another similar powered 4 cyl car like I said my old car had much more loss in winter than the Prius does.