The upper fin is bigger in size and shape than the lower fin. Another difference lies inside the jaws. The fish has teeth used for grasping prey. The shark however has larger, more broad teeth used for shredding and tearing meat. The third difference is, of course, in the skeletal system.
Sharks have the cartiliginous skeleton while the everyday guppie has bones.
The answer lies in the size difference between the two animals. In the ordinary fish, there is less surface area. Less force is needed to move the fish in a forward direction. Because of the sharks broad frontal area and immense size, a much greater force is needed to move the shark. Having the light skeletal system reduces the weight tremendously and less force is exerted.