My Taiga -18 down bag gets about as small as a half a 5 gallon plastic pail (using a compression sack...and pulling the straps to the stops) if that helps you visualize the size...maybe slightly bigger. The diameter is not much bigger than the bottom of a 5 gallon plastic pail and the length is slightly over half that of a pail.
I'm sure other bags get smaller but I just can't afford double the price for less than a 1/3rd better compression. I found it to be a fair compromise and I like the bag....comfy and warm!
And having a bag that might not compress as small as others takes away the temptation to used the extra space for packing stuff I really don't need!