Actually the current Bronx Amur tigers are in compatible small groups that are exhibited together, with no animals being housed in the holding spaces during zoo visiting hours. That said, both the indoor and outdoor off-exhibit areas are quite spacious (bigger and more fully enriched) than many "on-exhibit" spaces in other zoos.
And Black Rhino is correct that rotation of animals between off-exhibit spaces and on-exhibit is a very common practice. Some would argue this in fact is enriching, as animals get to experience different environments rather than living always in the same space
@reduakari: what is the exact size of Tiger Mountain at the Bronx Zoo? Is the exhibit space actually 3 acres? How large are the combined off-exhibit areas? Are there 2 tigers for each exhibit?