Though some cells are bigger than others, they do not scale with size. For example if you look at skin cells on a dog they would be similar in size to a skin cell from a human. When an animal or species is bigger it is usually just made from more cells.
Energy can always be transformed from one form to another form.
E.g. Throw a ball in upward direction ,so it will have some kinetic energy due to its mass and velocity. At some height ball will stop and will start coming down.
