Most of the energy expenditure of the body goes towards various maintenance tasks with can be regulated up or down. Brown fat tissue can even simply produce heat without any real process.

If a tissue is under high pressure it kills some of it's cells via apoptosis to lower the pressure under which each cell happens to be. If the tissue is under low pressure it increases cell numbers to fill up the available space. Of course available energy has some effect on this process as well but the body can regulate hunger or some other process when it thinks that some tissue really needs a different number of cells because it's under a lot of pressure or there isn't enough pressure.