How does that work?
Well, rough figures but if you're already pulling in a crowd of say 50k and expanding the stadium by 10k is going to cost you £10m (if you're lucky and Sir Bob built you an expandable stadium rather than have to rebuild completely). Unless you're already killing it at the top of the league, you use that capacity about half the time and need the tickets to be quite competitively priced to do it. That's maybe £2m a year additional revenue.
That £10m could have been invested in business development deals that dwarf that kind of revenue. With a bit of a top up of the wage and transfer budget thrown in. You could be making £5m a year more on sponsorship and other business deals, and making £5m a year just for finishing 3 league positions higher than you did before, and £8m for getting 5 more live games on the sports channels than you did before. And the stadium's already full.