Also can depend on regulations with regards to refueling. Firstly l'm not sure if SIA takes on additional fuel in MXP to fly to BCN or carries all from SIN. If they do take on some fuel, there are rules in each airport/ country if pax can be onboard or not. Some places allow pax to stay onboard if fire brigade is standing by the aircraft. That costs extra money though so in such places airlines often make pax leave the a/c. And so on. YMMV.