Bill's right: the tank still contains 3/4 of its fuel.
You might be confused about how that relates to the first half of the sentence. If you poke a hole in the bottom of a gallon of water, it will leak quickly when the gallon is full, and it will leak more slowly when the gallon is almost empty, because there is less water pressure. So, if the tank is leaking quickly, it must still contain a lot of gas.