Here is a little story to illustrate the point:
A scientist discovered a way to grow the size of a flea by several orders of magnitude. She was terribly excited. After all, a flea can jump vertically more than 30 times its body size. She reasoned that a flea this big would be able leap over a tall building. Perhaps, there could be a Nobel Prize in this.
When the day came to show the world, she pushed the button and sure enough out came this giant flea, over two meters high. But, rather than leaping a tall building, it took one look around and promptly fell over dead. Turns out it couldn’t breath. No lungs. Passive air holes that worked fine for oxygen exchange in a tiny flea were useless for a creature so big.