Well if we are going to get into good discussion we might as well argue the facts, right? Here are some October 2005 numbers for the NYC Metropolitan Area Airports...from the website that you quoted above.
Total Passengers
Continental - 17,679,253
Continental Express - 4,318,041
CAL Total - 21,997,294
American - 14,974,731
American Eagle - 1,635,857
AMR Total - 16,610,588
Looking at CAL's total numbers versus AMR's total numbers...CAL is 32.4% larger on a total passenger basis for October 2005. Where do you get 15%? Even if you use just CAL mainline versus AMR mainline it is an 18% difference. But I don't think that is the best way to analyze it since both Eagle and Continental Express are part of each carrier's network.
You mention that EWR has room for growth at Terminal A for AA (but no CAL growth at EWR)...have you not noticed that CAL has one of the fingers of Terminal A and has grown substantially there this year? All of CAL's Dallas, Boston, Atlanta, Chicago, Washington-Dulles, and Washington-National flights are now handled out of Terminal A. And Terminal C can grow easily...simply by shifting Continental Express flights to hardstands and moving the mainline flights to C2.
Being someone who currently lives on the east side of Manhattan and who grew up in Westchester County, I know a little of what you speak of...yes, CT and Westchester people generally do avoid Newark just as Northern NJ people avoid LGA and JFK. But frankly, I can't stand going to JFK (and most of my NYC friends can't stand it either). EWR is much more accessible in my opinion. Of course, if I lived in Dix Hills, Long Island or Greenwich, CT and wanted to fly nonstop to Omaha NE or Quebec City, Canada....besides Continental, what are my options? Southern Westchester isn't THAT far from Newark by the way...40-45 minute drive versus 30-35 minutes to LGA and you cross a bridge for both.
-Neal