I should think it would relate to the habitat within the space. Royal antelope are forest antelope (and nyala bush antelope) so they would not be comfortable in large open spaces. Fill that space with vegetation and they would be fine.
To me the argument (particularly used by reptile keepers) that animals "need" small enclosures to make them happy is just used as a justification.
What are some other US zoos with this species? The only other ones I'm aware of are Brookfield and San Diego. Does Lowrey Park even still have the species?
@pachyderm pro Not sure when they left but the photo above was taken in mixed African aviary (hornbills, spurfowl etc) opposite the entrance to the safari ride.