Which would you prefer to drill, a 500,000,000 bbl prospect with an 80% chance of success or a 2 billion bbl prospect with a 20% chance of success. In both instances the risked amount of oil would be 400,000,000 bbls of oil. Most companies will take the 80% chance and get some success under their belts first. Also a disovery on a "smaller" prospect may increase your odds on the other one. That is why companies will sometimes drill what appears to be a smaller prospect.