Most stuff that moves keep the same volume as it moves. For many fuel sources, fire doesn't work this way because it is not quite the same physical matter in a form of a gas, liquid, or solid. You can get a convincing fire by making it grow and shrink in waves.
At this tiny scale, animating a fire shouldn't be terribly difficult... it should probably take like 3 min of work or something for something around the size of the torch fire. Probably exaggerating but you get the idea. I don't think a huge fire should be 'difficult' if I know how a fire works to get as far as I did with this study.