How much space does a lion need in captivity?

Lions in captivity don't need to hunt because they are fed. But cages are like prison cells. The lion enclosure at the Toronto Zoo is a couple of acres outdoors and about 1500 square feet indoors. The indoor enclosure has a part that is not visible to the public and the lions go in there. But they are curious enough to have a look at the public too and they are fed in the publically visible area.