Well, I consider my current 3+ pound REI down bag and my synthetic 4 lb bag heavy. That's cause I see lighter bags for the same zero temperature ratings advertized all the time. However, my experimenting with lighter bags, while limited by funding has left me skeptical that lighter weight can be warmer. Weight seems to equal heat in a sleeping bag. I have spent some miserable nights in lightweight bags. A tip is that a top quality insulated pad is necessary for anything below freezing.
