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... etc etc
Except in Cataclysm, the amount of combustible matieral is more closely tied to volume than to weight. Density isn't really tracked.
It... doesn't matter if density is tracked. Five pounds of material is five pounds of material. It doesn't matter what the volume is. I'm getting really confused trying to figure out how you think chemical reactions work. No matter what the relative densities of materials, five pounds is still five pounds. The only other difference would be the heat of combustion of the material, which would be in terms of mass anyway.
The way you're doing it, when you have two items of equal mass but different volume, the higher-volume one burns longer and with more heat. In reality, the higher-volume one shouldn't burn as long at all, because it likely has more surface area, and they should burn with the same amount of heat in the end.
The amount of combustion heat you get out of something is related only to the mass/weight of the object you burn and the heat of combustion of the material in terms of energy-per-gram (which obviously isn't tracked here, so we have to assume it's equal across the board).
And if volume in Cataclysm really is volume in the scientific sense, then density is tracked. If you have volume and mass, then you have density.