Most down jackets I am familiar with easily compress to a size significantly smaller than a pillow--grapefruit size is more like it for some. The WM Flight or Nunatak Skaha STUFF down to the size of two grapefruits, for example. Put them in a compression sack and a 6" x6" x 7" ball is what you get. A comparable down vest would generally stuff down to a smaller size. For instance, a TNF Nuptse vest I have stuffs into its own pocket, which is about 9" x 5" x 6". I'd think that any down jacket made of UL materials and using high f.p. down would have a small compressed size.
I think you'd better be more specific, though. The down jackets I am referring to above are summer-three season appropriate, and while they'd certainly help on a snowshoeing day hike, they are not winter jackets and will not cut it in sub zero conditions. Maybe take a look at the Feathered Friends Volant jacket or Frontpoint jacket. I have a Frontpoint in Flite nylon and although its stuffed size is about 16" x 8" x 8", it compresses to about half that size.