The SQ "lay down beds" are not all that flash, having tried to stop myself sliding to the floor for many hours on a sin - BNE flight.
The service is great, the food good, but the Seats are nothing special.
Not sure that a 737 has a lower seat mile cost than a 330 so a full flight on a 330 should generate more profit than a full 737, the clincher is whether they can fill the 330.
If they can't - and they get say 180 on average per flight, then a 737 will be far more profitable, as the sector cost of a 330 will be much greater than the sector cost of a 737.
The other part of the equation is what else they could be doing with the airframes. If the 737 is already in Perth and doing a lot of sitting around because of the schedule, and the 330 isn't, then it is biased towards using the 737.
It also depends on when the flights depart and arrive. Are they scheduled to sync with domestic flights from inter and intra state? Do a bunch of miners on FIFO arrive in time to connect with the flight to Singapore or does the flight from Singapore arrive in time to get someone coming from Asia onto a FIFO flight to a mine site.
You can't look at the individual city pairs in isolation.