Take it slow!
A full stadium gives your team more possession, in home games obviously. The advantage of doing small upgrades is that you can benefit from the extra income (small though it is) sooner, but possibly more importantly it gives a chance for your attendance to grow gradually.
A full stadium build takes 16 days, you can't build anything else in the interim and suddenly you have to find fans to fill that increased capacity.
When are we going to see the options to make our stadiums useable on non-matchdays? I want to hold weddings, 1st communions, wakes, all sorts in mine.