Mike -
Pellet length is determined by several factors that I have observed, and I am sure there are some that I have not...
Wood species is important because some woods have more lignans (naturally occurring polymeric compounds found in cellulosic fiber, i.e. plants and trees.
And - it seems that darker, more dense woods contain more lignans...mesquite, and black walnut pellets come to mind.
Those kinds of woods tend to make pellets that can run longer, because when pellets are made, the sawdust is pressed, or squeezed under tremendous pressure, causing the lignans to become semi-lquid or syrupy, and glue the pellets together better.