Some fleas have to accelerate to over 140 gravity forces, or 50 times the acceleration rate of the space shuttle, in order to jump just a few inches into the air. This sounds impressive but actually the stresses in such a small animal are not particularly high, and it is the stresses that matter. If a flea grew to human size it would probably not be able to accelerate as fast as us. Two things make it hard for small critters to jump very high. The first problem is air resistance .
Air resistance slows small things a lot more than big things.
For an animal the size of a flea, air resistance is a huge problem. Nothing can be done about this except, of course, go somewhere where there is no air, like on the moon. The second problem is that muscle moves too slow . The flea's short legs only allow it an acceleration distance of a fraction of a millimeter. There are real physical limits on how fast muscles can move and how much power they can generate.