Just one last point - even if the case can be made on a percentage basis that regular cars don't take the hit a hybrid does in winter, in an absolute measure, hybrids will still out-perform any other (or...a vast, vast majority) of vehicles in the winter. A hybrid getting 36 mpg in the dead of winter is still better than 99% of the other vehicles even if you give them pleasant sunny day on a flat road with the wind to their back.....
I'd add a third reason - a 5.0 litre V-8 is going to generate a LOT more heat than my little 1.3 L - makes sense the smaller engine would be more sensitive to cold....