From http://www.apple.com/ipod/specs.html:

Size and weight:
Height: 4.02 inches (102 mm)
Width: 2.43 inches (61.8 mm)
Depth: 0.78 inches (19.9 mm)
Weight: 6.5 ounces (185 g)

From http://www.riohome.com/audio/rio/rio_riot.asp:

Player Dimensions:
5 3/8" (136.5mm) x 3 5/8" (92.1mm) x 1 /3/8" (34.9mm)
10 oz. (including battery)

(italicized mm calcs mine)

That means that the iPod is 25% shorter, 33% narrower, and 43% shallower. The iPod is 125.44cc in volume. The Riot is 438.75cc in volume. That's 3.5 times bigger. (That can't be right. I've never seen one, but the Riot can't be that big. Someone correct my math.)

Drawbacks to the iPod include that it's got only one quarter the data capacity of the Riot (5GB vs. 20GB) and it's only officially supported on the Mac platform, although Windows software does exist. And you might consider FireWire-only a drawback, or you might not.