This jacket is very fitted so you don't look like the Michelin Man in a white puffer jacket.  When they say M is size 8 - 10, they mean a fitted 8 - 10.  If you want to wear a heavier sweater consider sizing up.  I chose this for an easy jacket for shopping or walks.  It is warm but not stifling. I use in temps hovering around the freezing mark with a fine wool sweater and it works.  I am always cold btw. I wanted a white puffer that was fitted and that I could throw in the washer because it is white.  I would have preferred actual down instead of fill but in the end chose this one because of the fitted cut.  I am satisfied with the choice.