I've always thought that it was the 800-pound gorilla, so I thought that the OP just made a mistake, but no, the 900-pound gorilla is a thing: an article (http://articles.chicagotribune.com/2011-01-05/features/ct-tr...) in Chicago Tribune talking about the origins of the 800PG phrase has this:

Sometimes the gorilla surpasses a mere 800 pounds. ("Whose the 900-pound gorilla now?" asked a headline on a recent tech story about Facebook overtaking Google as the biggest web site in 2010. Sometimes the gorilla sheds a few hundred pounds. ( Colorado's governor-elect was quoted last month calling the state's billion-dollar shortfall "the 600-pound gorilla.")

For some reason, the 900PG is common in tech posts, this may be due to the effect of a rock band with that name. We may be witnessing a the emergence of a differentiation like the soda/pop one.