In reply to Birks:
I've got a Mountain Hardwear Hyperlamina. It packs very small for a synthetic and is lightweight and warm. I spent ages trying to find a synthetic that had small pack size and this was definitely the best choice for me.
It is a fairly slim bag with a centre zip. I'm a slim but tall female and it is comfortable for me, but the bag may be a bit snug if you're taller and broader.