I can think of a number of new hippo exhibits which are significantly smaller so that cant be it... Dallas for one has a tiny land area, less than half the size and its a brand new "state of the art" complex. From what I read part of the reason was the amount of water needed to drain the pool multiple times a week, although why thats a problem in the pacific northwest is beyond me, its not like we are a drought region.
The Zoo’s 2002 Master Plan proposed a new Hippo exhibit with a larger holding area, land space, underwater viewing and a much more sustainable filtered water system. It would have been located approximately where the current giraffe holding building and yards are, so they would have to be relocated first. Obviously, funding priorities (and realities) have changed since then. it’s too bad—the original concept and execution of the current hippo exhibit as part of the Savanna complex was brilliant.