My Stadium is not full (I have more seats than it is shown)
As it is in real football, every new club needs to grow up first and attract more fans. It won’t be realistic for a new club to get a massive stadium and a lot of fans from the start, when is not that famous still, so there is a fans limit per level in the game. So, as for level 1 the limit is 800, for level 2 the limit is 2500, for level 3 is 6000 and so on. Your Club level is the factor of your stadium capacity, no matter how much are you going to upgrade it. Still the stadium construction takes massive amount of time, so upgrading your stadium from now will be in great benefit for you later, so don't worry!