Although I have only been to Zenkaikon for 2 years, even in a year's time it has grown a lot bigger. This year it was Zenkaikon's 5th year running and it grew from being in a small room to 2 floors of the Valley Forge Convention Center.
Unfortunately, the massive growth wasn't without some growing pains. Last year when the convention took place inside the Radisson Hotel next to the Valley Forge Convention Center, it was very crowded. The crowding made the small con experience feel like a big con despite having small con guests and events. Fortunately, the change of venue to the convention center solved the crowding problem and allowed the convention to grow comfortably to several thousand attendees. In addition, the convention is still in it's sophomore stage as the staff are still unable to work the A/V well enough to allow for panels to start on time.
The high growth can be attributed mostly to the location. This is the only anime convention in the Philadelphia / East Pennsylvania area. In addition, it also attracts con goers from New Jersey and even New York. Another anime convention was being organized to take place nearby, but these plans were eventually scrapped.
Of course I have to mention the earthquake/tsunami that ravaged North-East Japan because Zenkaikon did their part in raising money to help those in need.