You will lose more % in a hybrid in winter.

All gasoline engines are worse in winter.

In addition, hybrid batteries are worse in winter.
Cold batteries put out fewer amps, and cold batteries can take in fewer amps of regen. So you get a double whammy.

But think about where you started from in the summer, and you should feel better.

Losing 25% of 30 MPG (hybrid) is still better than losing 20% of 15 MPG for a non-hybrid.

-John