That could be the case. Averages are pumped up by bigger runs. However, it can be especially true with a lower amount of carries. I remember when fans wanted to start Troy Hambrick over Emmitt because Hambrick's average was higher as a backup with a whole lot fewer carries. Emmitt left, they tried Hambrick as the starter and his average tumbled.
I personally prefer backs who can average 4-plus without any big runs because that shows they are not really just averaging a yard or two and every now and then inflating their YPC with a 20-yarder. I mean, if you had four runs for a yard each and then ran for 20, you're averaging 5 YPC.