Pay isn’t about size, weight, passenger capacity, routes, complexity of the aircraft etc.
It comes down to legality and free market economics.
Does the contract comply with the applicable legislation? And then to supply and demand, will there be enough qualified and willing workers to perform the job under these conditions?
If there aren’t then you have grounds to improve conditions. If there is an oversupply then reductions and cuts will be sought.
Do you think mining companies want to pay six figure salaries for FIFO cleaners? It’s because they need to, not out of the goodness of their hearts.