Update (8/11/11): I got a few questions about the source for Doc Ellis saying the catcher's mitt looked bigger at times. The only primary source I know of for this incident, other than the game itself, is an interview with Ellis about the game. In it he says that sometime the ball changed size. It is a minute detail, but I was still wrong.
The magazine Scientific American Mind has a small article on a study where students were asked to putt a golf ball into a hole. When asked to estimate the size of the target, students who had successfully sunk the ball described a bigger target.
This is in line with what a lot of baseball players claim to experience before they hit a home run, they report the ball being huge. When Doc Ellis pitched a perfect game on LSD, he reported at times that the catcher's mitt seemed enormous.
It is important to note though, that visualizing a bigger target is only correlated with being able to hit it. So while it might be possible that because we perceive a bigger target we are more likely to hit it, it is just as likely that once we hit the target, we remember it as being bigger.
At the bottom of the article, Scientific American's site let's you buy the rest of the article. However when I went to my local grocery store, and picked up a copy of the magazine, there was nothing more to the article. If you want to buy a copy of this magazine, don't do it for this article, but do purchase it because there are tons of other articles pertaining to thought, cognition, and the mind.
Anderson, Andrea. "Towering Targets: Why the Ball Looks Bigger When You're on Your Game: Scientific American." Scientific American. July 2011. Web. 06 Aug. 2011. <http://www.scientificamerican.com/article.cfm?id=towering-targets>.