I guess most of the chimps and organgs used in films are typically very young and (in the case of orangutans) female. I was quite surprised when I first saw full grown chimps at a zoo (they stand over five feet tall), and full grown mail orangutans are not only huge (like in the 400 to 500 pound range, I think), but also have giant, wing-like flaps of skin on each side of their face.

The Dead Next Door: A Field Guide to Regional Horror Films
Regional Horror Films, 1958-1990: A State-by-State Guide