A mouse weighs just under an ounce; an elephant 11,000 pounds. In other words, an elephant weighs 250,000 times more than the mouse does. But there's a problem here. As a creature grows in size, its insides grow faster than its outsides.
If a little mouse were to grow 250,000 times bigger to become an elephant, once it gets elephant-sized, it has trillions of warm cells bunched together in its body, so it's pretty hot in there. And, because it's grown bigger, it also has more surface area to let the heat out. If an elephant burned fuel at the same rate as a mouse, its insides should get so impossibly hot, that at some point, it should just... explode.