What the other guys said is true, tent makers lie about capacity. The trouble with a big tent is finding room to set it up. If you are just going to use it in regular campgrounds this is usually not a problem, or if you live in Kansas or Nebraska where everything is nice and flat, no problem. But if you take it to the mountains in lots of places it may be a real problem to find very many square feet of flat level ground all in one place.
To make matters worse, in lots of the nice places where you could maybe squeeze in a good size tent, the people who have gone before have thought that a nice fire ring with big rocks full of ash and half burned trash is just what needs to be right in the middle of any nice clear area.