What Walt says is absolutely correct. The Spindrift was designed to be worn UNDER an outerwear layer when hunting, both for noise suppression and protection of the thin, slick material covering the Spindrift. The jacket is thin, light, compressible and warm and perfect for its intended purpose.
With respect to fit, the Spindrift runs true to size. I normally wear size large, and that is what I wear in the Spindrift. The Spindrift is similar in design to the Sitka Kelvin jacket and the Patagonia MicroPuff jacket except designed better for hunting when worn under an outer jacket. I have all three brands of these Primaloft insulated jackets, so I am not saying this without an adequate comparison of the three of them, all of which are very nice jackets. I also have two North Face Redpoint jackets which are insulated with Primaloft. They are nice too, but are not as thin and minimalist in design as the other three brands. I don't think you would be disappointed with the Spindrift.
Allan