See this is the conundrum. Some pilots flying in common category loathe having separate pay on aircraft (M88-M90) so the lower paying equipment has been brought up to the higher paying rate. There is some downside though, as the higher paying equipment does somewhat subsidize the higher pay rate of the smaller equipment.

Now when you introduce a new piece of equipment that should generate more revenue and justify higher pay, we want it to pay more than the current category, but how long will it be before pilots start complaining that they aren't getting paid as much to fly the 320 as the 321?

This is one of the problems with pay banding.