Why is my stadium capacity stuck to 800

[Staggy Stagg]

The number of supporters is dependent on the level you are and not the stadium capacity.
Level 1 is 800
Level 2 is 2500
and so on. There's a list somewhere in the forum around page 10.

[iammrmehul]

I have seen my opponents having 1000 capacity as well, they are also level 1

[Pangihutan Manik]

I have seen my opponents having 1000 capacity as well, they are also level 1

Hmm.. Maybe it's a bug. You can post it in the bug and issue list with screenshot of the opponent and you.