Great question. The optimizer is a pretty complex bit of software, I'm not 100% on how it functions but it is less accurate the larger the search space it is given. The best method to use it would be to run a simulation with it set to the high value and then reduce it to something more reasonable for accurate results. In your scenario I would see what happens if you run it at 2000 kWp and run with those results, as in the 10000 kWp scenario suggested a value greater than your 1000 kWp simulation.
Hope that helps,