The Rams' running backs outweigh the Seahawks' running backs by more than 17 pounds on average, not counting fullbacks.
Average weights by team: Rams 227, 49ers 220.5, Cardinals 211.2 and Seahawks 209.8. The Cardinals' average reflects returner LaRod Stephens-Howling, listed at 185 pounds. Seattle's average would be higher if LenDale White weren't in shape.
Using multiple backs in rotation should allow teams to carry runners of various styles and sizes. Carrying smaller backs also becomes easier when teams use several instead of relying disproportionately on one.
The chart breaks out the runners individually using listed weights. Jason Wright has played fullback for the Cardinals, but he's more of a utility back than full-time blocker.