The flights ex LAX to MEL typically take over an hour longer then those ex MEL to LAX. This is due to the prevailing jetstream winds which are generally westerly in nature (west to east).

Plucking a date at random with said 388 flights, on September 15th the flight times were 842 minutes for QF93 and 952 minutes for QF94.

So significantly more fuel is required for same distance - hence more mass to accelerate to airspeed upon takeoff.