I think the idea is to encourage the Orangs to stay off the floor. There is a lot of vertical space for the Orangs to climb around in which makes up for the lack of floor space.
The bonobo and gorilla land areas are bigger because they spend more time on the floor than Orangs.
If the indoor area does as , I understand from Toddy's post act as a hub with all the out door space radiating out and open at all times it makes for an even better exhibit. If there is one who want so move out on their own they can get away easily.
A great exhibit my only question would be how long the tree trunks in contact with the ground will last before rotting away.