Its quite simple really.
If the aircraft has 280 seats, 50 of which are J class then the aircraft sells tickets at a premium both in Y and J class in QF.
It therefore can pay a higher price to its tech crew proportionate to the revenue/premium received.
Same goes for an A380 with 400 plus seats.
So you cannot compare LCC pay with Full service airlines. Just like you can not compare metro pilot pay to that of an A380.
It must be a like for like comparison.