PaulC - Chips would burn way too fast. If you reduced the wood to a fine powder and aerated it, it would explode. At the other end of the spectrum, a big solid piece won't burn worth a darn. Has to do with the ratio between mass vs. suface area. If its important enough to anyone, I'll try to dig up a research paper that resides somewhere in my "fantasyland-type" filing system.
(Don't hold your breath, though. I've succeeded in hiding some stuff so well that I gave up trying to find it....)

Cheers,
Wes