Why do penguins only have small flipper-like wings?

Penguins - nearly all birds use their wings for flight, but penguins use theirs as paddles for swimming. They spend most of their lives in the sea and find their food there. Although many birds can both swim and fly, no other bird can swim as well as the flightless penguins. Penguins have muscles, bones and organs very much like those of flying birds, so we assume that their ancestors must have been able to fly. Probably they slowly lost the power of flight while learning to swim faster and dive deeper in search of food. This must have happened millions of years ago, for by Miocene times 25,000,000 years ago-there were penguins very much like those alive today. The feathers of penguins are short and grow all over their bodies, leaving no bare spots unprotected from cold air and water. The feathers of the paddle like wing are small and stiff, with broad flat shafts. On the rear edge of the wing are rows of many short feathers with strong shafts. The bones of the wing are flattened. This allows the whole wing to be thin and streamlined, but yet strong enough to push the bird through the water. Its shape is much like that of the bones have grown together, so that the penguin paddle is stiffer, though thinner, than the wing of a flying bird. The breast muscles, which work the wings, are as large and powerful as those of any bird, but the muscles in the wings are small. Many of them are largely made up of slender bands of strong tendon. This, too, helps in making the paddle wing flat and thin.