There are lots of variables...
the 'look' the designer wants to acheive, some bikes have even been designed with a dummy silencer on one side to balance the look of the necessary silencer on the other side of the bike.
the requirements of the engine, eg how much hot exhaust gas it has to pass (diameter), how much back-preassure is required at the engine out-let port (length).
it's fit (to the engine and the chassis) and weight ( as a component and over-all on the weight distribution of the vehicle), if a Catalytic converter is fitted.