Durability is the main factor in material design apart from strength. In case of natural fiber durability is the most important aspect because natural fibers are sensitive to the ambient conditions such as temperature, wetting-drying, freeze-thaw etc..

In my opinion the most important factor when choosing natural fibers is their slenderness (ratio of length to diameter or width). The strength of the individual fibers, such as those of the timber (ligno-cellulosic fibers) is much greater than the strength of the material, such as MDF for example.

So from contact, active area, depends the amount and the  type of the binders between the fibers and hence the strength of the material.