There may be a physical difference between Steven Jackson and Daryl Richardson, but there are certainly certain aspects of their game that are similar*. Just take a look at the two videos and you will see them two looking a little similar.
*From the small sample size we have from Richardson.
Steven Jackson Runs (via choober7 via JoshZachary24)
On the long runs we can see how Jackson maneuvers his ways to the outside and then turns on the jets to burn everyone else. On the short TDs, Jackson takes contact and then powers through it to get in. He seems to make sure he runs hard enough to get to the endzone
Darryl Richardson 4yd TD run vs Kansas City (via choober7)
On Richardson's long run against Washington, he also gets to the outside. It's impressive for a rookie to keep his speed and balance when contacted as he is. He also shifts into high gear and comes up just short of the end zone. On his short Touchdown Richardson hits contact quickly, but he is able to push hard enough to dive into the endzone.
Jackson in Richardson could be a great pair of backs. I feel like Jackson has more power and is harder to tackle, but Richardson is quicker and hits the hole just as hard as Jackson. The two runners together eventually could be a hard part of our game for defenses to stop.