The CFM 56-7BE (on the 737) has a long and narrow nozzle, the RR Trent 900 (on the A380) has a sharper nozzle, and the CF-6 (on the 767) has a wider opening at the nozzle. Why do the nozzle sizes vary among turbofans?
The nozzle, apart from its main function, that is accelerating the air in order to propel the vehicle, it determines what goes into the front of the engine, it is a downstream restrictor along with the turbine nozzle.
The areas of both the propelling nozzle and turbine nozzle set the mass flow through the engine and maximum pressure. For different engine operational purposes, it is used one shape or another. These engines have different shapes due to optimization reasons; the engine, is designed in such a way that the aircraft features will have the best balance in terms of fuel consumption, thrust needed, and many other parameters.
Each manufacturer design their engines in the most convenient way and then, it is determined which engine is better for a given aircraft.
Different kind of geometries, (e.g. convergent-divergent) also affect on the engine operation regime (supersonic, subsonic, transonic), but this does not affect here since all the engines are turbofans and thus, subsonic.