I guess this depends upon whether the "footprint" the or cubic volume is most important to you. For my purposes, I chose length and width, as the more important consideration:

Rio Karma 20GB = 2.7"(l) x 3"(w) = 8.1 square inches
iRiver iHP-120 = 4.13"(l) x 2.36"(w) = 9.74 cubic inches

From this perspective, the Rio Karma is almost 20% smaller.