Using the tire sizes you've specified, and checking the specifications on them, the 20" rim diameter tire weighs an average of 6.5 pounds more than the comparable 15" tire of the same outside diameter. You do have to keep in mind that as the tire sidewall is shortened, the load capacity id diminished. In order to keep that load capacity the same, the tire has to be built with thicker sidewalls in order to keep the capacity the same as a smaller wheel size tire of the same outside diameter.
