Why do smaller pieces of wood burn faster?

Answer 1

I take it you mean faster than large pieces of wood.

In essence it is down to the larger surface area to volume ratio of the smaller piece of wood. If we had a 1 metre cube of wood the surface area is 6 m^2 and the volume is 1 m^3. If we divided this into 8 equal cubes, each cube is now 1.5 m^2 (6 x0.5 m x 0.5 m) in area so the total is 12 m^2 and the volume is still 1 m^3.
Thus the air sees a much larger surface area and the wood will burn faster than the original cube.
If we continue this until we reach saw dust we have a very large surface area indeed. So much so that saw dust in air will burn so fast that it can cause an explosion.

There is quite a good demonstration on YouTube search for "Jake's Attic - Saw Dust Explosion"

Answer 2

basically.......cause there is less of it to burn... :)