In reply to JayPee630:
I recently also bought a new synthetic bag, and the one that seemed to top the list was this one:
http://www.mountainhardwear.com/hyperlamina-spark-35f-1c-regular-1568251.ht...
737g, 1 degree temp. My partner bought one, and it's really quite good (I haven't slept in it thought - not allowed to!).
In terms of pack size, it packs down similar to about watermelon size thereabouts (rather than bucket sized).