In a recent interview (D8 Conference), Jobs called Apple "the world's biggest startup". I don't think it's a question of size, revenue or growth, but rather the mentality of the company and the individuals working there.
The word is "Start" + "up". Time probably has the most to do with it, because your not a startup after more than a couple years. If your still small after that long, you're just an underperforming company. Growth probably has a to do with it as well, because the bigger and more successful you grow, the closer you are to reaching your company's maximum potential. Tons of employees aren't typical of company's that just start.
So there's my logic!