why I'm growing to hate colder weather!

I've been exercising (running and now walking more with age) for over 20
years. I've always been perplexed why I seem to gain about 10-15 lbs
during colder weather than warmer, but this year I think I'm beginning
to figure it out. I eat pretty much the same way all season long but I
exercise outside and keep a colder house. I notice I don't sweat as
much obviously, and it takes more to attain the heart rate needed.
Based on this I'm going to say that it takes more exercise to burn the
same amount of calories during colder weather plus I don't sweat as much
and don't manage to lose as many "water pounds". Not sure if it makes
any sense, but this is what I have come up with. Right now, I'm about 7
lbs above where I was mid-Summer and this has been with exercising about
2x as much and I'm not really eating anything different. Thoughts?