We have 2 for the kids, they fit a whole junior ready bed inside. They do feel quite thin. We are fair weather campers, I'm not sure how thick and cozy they would be if it got chilly.
I have a Gelert one, from the 'Beyond' range (bright blue) which is heavier and a three season one, unlike their normal ones which are two season. I prefer the ease of movement (side sleeper one knee up!) but the pack size is about half as big again as s regular mummy bag.