Largest convention center in Europe Im told.
Actually, "just" the largest convention hotel in Germany, and at least one of the largest in Europe. The term "convention center" usually includes exposition grounds and halls that haven't large hotels attached. Of those there are many that a are a lot larger than the Estrel complex... Which is a problem for fan conventions in Europe: a lot is built for huge trade fairs but not much for large social events.
America has a lot of those convention hotels due to family reunions and gatherings of religious groups every couple of years that draw a large number of attendees who naturally want to stay together as close as possible due to the enormous travel distances — especially before paved roads existed across the board and commercial air travel became a commodity. Travel was strenuous and expensive, which made every second spent with your peers very valuable. I believe that was the main factor for conventions becoming primarily social events in America.
Europe never had such a tradition. Families mostly stayed together in one region. Most people belonged to one of the few large Christian denominations. Trade fairs were for business only, at most you met a few of your business partners at a hotel bar at night, but that was that. And thus, many fan conventions in Europe are either organized as trade fairs or very small. Though that has changed a lot in the last 15 years or so, something I really appreciate.