There is a rule for calling yourself the "tallest building" that is relevant here. You aren't the tallest building until you have topped out. Before that, you are just a construction site. This is important because it means you cannot be "tallest" while still under construction. The title belongs to the tallest current building until such time as you stop building.

This "worlds biggest" solar plant therefore isn't. It isn't yet biggest. It may never be biggest. It may be passed over by some other plant started tomorrow. Or it could stop expanding before ever reaching its designed size.