machka, unfortunately, itty-bitty & light down bags = cost a fortune. luckily, that's only when you actually shell out the money. although i only use it as a loaner now, i've got a winter down bag that i bought in the '70's that's still more than servicable. i paid the astronomical sum of something like $80 or $90 i think. comes out to less than $3 per year. i've had 3-season and winter bags from rei & northface, but now i go exclusively with feathered friends bags. in simple terms, it's like the difference between a custom bike and a walmart bike. western mountaineering also has a very high reputation, although i don't have any personal experience with their bags.

synthetic bags will lose quite a bit of loft and start leaving you cold after 4 or 5 years with average use, maybe 3 years with extensive use and using compression straps. you'll be losing money in the (not so) long run.

someone suggested using a space blanket to reflect heat back to you, but it'll actually do diddly squat. if you want to sleep on top of snow and ice, you need a thick layer of insulation, no two ways about it. there's no way to avoid bulk on this one. the lightest option is mec's thickest evazote pad. if you want to go thinner, take all your clothes out and sleep on top of them too.

oh, and by the way, after having skidded out on icy patches more than a few times on one trip, i now limit my winter trips to only a few days when i can be sure there won't be any new snow coming.