No, volume is more appropriate here. Weight doesn't really reflect an items density, and the amount of combustible material is more closely aligned with volume than weight.
I'm not sure what you mean. Amount of material
is weight. Well, mass, not weight, but those are identical for our intents and purposes. Neither weight
nor volume reflect an item's density, both because 1) density is a property of both those things combined, and 2) more importantly, "volume" in Cataclysm is the "bulkiness" of an object, it seems, not just its actual displacement volume. In other words, a solid bar of iron has less volume than, say, a really long pole made out of it, or a bundle of chain-link fence.
I just don't possibly understand why the amount of combustible material should be described by volume, because that's not how physics or chemistry actually work. The amount of time something combusts, and the total heat (as well as heat per unit time) given off is related to how much material there is combusting in mass. Total amount of combustion heat correlates positively with mass/weight, and how quickly it combusts correlates positively with volume (or, often more relevantly, surface area, but we don't have a notion of that in the game). If you have two items in the game of the same mass, the higher-volume one should burn more quickly (because it's less dense) but provide the same amount of overall heat. If you have two items of equivalent volume but different mass, one is more dense and therefore should burn with a greater total heat but possibly more slowly.
It's basic physics and chemistry, really. The volume of something has nothing to do with "how much" there is; mass is the appropriate measure of that. A larger item of less-dense wood will produce the same amount of heat as a smaller item of some denser wood, if they have the same mass (and the constituent compounds have similar heats of combustion, which is safe to assume). A foot-cubed log of oak will produce a whole lot more heat than a foot-cubed log of balsa, and the latter will likely burn more quickly.