OK....
So lets take a plane full of 50kg people flying SYD-MEL each paying $150 for their ticket.
And lets take a second plane of 100kg people only half occupied each also paying $150.
Same total pax weight, so same freight capacity.
Which flight makes money?
Fill up the second plane - and it is only adding 8t of weight (assume 737 160-seat) - What is the marginal fuel cost of this? And/or lost revenue compared to displacing freight.