The compressor running in a defrost cycle should not create a huge change in MPG.
Stop starting the car when you don't intend to drive it somewhere, as idling consumes fuel and if it's idling for no reason, there is no point. See what your MPG is after changing that practice. Then we can compare notes.

Also keep in mind, MPG is a direct effect of your driving style as well as a variety of other conditions.

 

I agree. I check my mileage at every fill-up and never notice a difference even in the summer when I use a/c. Sure, there's a always a slight variance (maybe 0.5-1.0 mpg) at every fill-up, but I get that whether I use a/c or not.