Does a loose fitting down/synthetic insulating layer compromise it's performance?
Case in point: I've just received my Cocoon UL 60. It's a great jacket, but in medium I'm a little surprised at how roomy it is.
Obviously 'loose' and 'fitting' are totally relative terms, but to try and put it in perspective: at just under 5'9" and stocky rather than thin (77kg/170lbs) I'm generally spot on a medium size in most clothes. For example my 2007 Montbell Thermawrap Vest in medium is close fitting - I couldn't wear a heavy-weight base layer underneath.
So I think I could easily go down to small in the Cocoon. The question is, should I? The extra room means I'd comfortably be able to fit an insulated vest underneath, without any risk of compression. Even with the looser fit, the lycra around the waist & hood and the adjustable hood still ensures a good seal. Weight difference between the two sizes in negligible.