idioms - Why does the gorilla weigh exactly 800 pounds?

It is common in American English to refer to a powerful person or organization as an 800 pound gorilla. The expression makes sense -- a gorilla of that size would certainly be intimidating -- but what's so special about the number 800?

Searching "n lb gorilla" on Google for various values of n results in many more hits when n = 800 than when n is any other quantity. Also, the Google Ngram Viewer finds notable occurrences of the phrase "800 pound gorilla" in texts starting around the late 1970s; other sized gorillas don't seem to be mentioned at all.

The Wikipedia entry for the idiom claims that the phrase is rooted in a riddle ("Where does an 800 lb gorilla sleep?"), but that's just passing the buck. A 700 lb or 900 lb gorilla would be equally entitled to sleep wherever he wants to.

Interestingly, the Wikipedia entry for gorilla states that the average male gorilla weighs about 400 lb, occasionally reaching 500 lb in the wild. Only obese gorillas in captivity have attained weights of 600 lb. So the number doesn't even seem to be related to gorilla biology (except that it is safely larger than any known gorilla, save King Kong).

Given that it's an arbitrary figure, is there any reason the number 800 would be more popular than any other? More generally, is there any relevant research into why some numbers would be more popular or pleasing than other in contexts like this?

Answer

The oldest reference I found in Google Books is Treks Across the Veldt by Theodore J. Waldeck in 1944:

... for there was almost no exaggerating the strength of a seven- or eight-hundred-pound gorilla, who could lift a man in one hand and pluck off his legs and arms as if they had been parts of a fly.

The next Google Books reference I found is from the 1960 The Geological Evolution of North America: A Regional Approach to Historical Geology by Clark and Stearn:

"As one branch of the apes increased in size to animals like the 800- pound gorilla, they found swinging through the trees difficult and began to walk on the ground."

These do not definitively answer "why 800?", but it's notable they're about real gorillas rather than the metaphorical gorillas who sleep wherever they wish, and show up from 1967 onwards (Challenge: the magazine of economic affairs, Volume 2):

The financial markets today are the equivalent of an 800-pound gorilla. They are difficult to ignore!