There's not a straight forward answer, but generally early in the game you want to get big quick and scare the other players off. Think when you're scanning down a set of destinations looking for a new juicy route. When you see one with 500 pax/daily but 2000 supply you think not likely and move on and keep looking! So you've been scared off. The big players will have some juicy routes that make them money, and they use this to subsidise some other routes that loose money. You hope the other guys give up first.
There's also in hubs the desire to grab slots, so players will run routes and lose money just to use the slots and deny them to others, whereas if slots were plentiful you may not run unprofitable routes.
A lot of this game is like poker, you make some blind guesses and push things to the limit and hope you come through the other side intact. It's all ebb, flow and bluff. A good player will spend as much time monitoring their competitors as running their own airline.
Also don't just look at seat supply, eg 500 pax/day and 1000 supply. It could be the supply is 2x747 at 500 seats each. In which case a competitor could put on, say, 5xA320 and soak up the passengers and the 747s would fly empty. It's as much about frequency as much as about size.
Simon