Since most people have no clue how to calculate gas mileage and base it on how many miles they do on a tank, it only makes sense to put a small gas tank in the Prius. Just imagine the horrors if people found out that their hybrid costs the same to fill up as a regular Civic or Cruze. Like someone already mentioned, it's about bragging rights, or in other words, perception. Huge gas tank just doesn't give that perception of fuel saver.