I think that McFlyPHL pretty much covered the gist of the issue....

The different pay scales for different airplanes goes back to near the start of the airline business when ALPA was formed and pay rates were first negotiated. For a long time the newer, bigger airplanes were more complex and thus "harder" to learn if not to actually fly - especially considering the state of navigation that existed and the bigger planes usually flew longer routes that depended more on the relative primitive navigation systems in existence then. So pay scales developed that considered size (weight) and speed added to a base rate which had a day and night component. Although much simplified now, the basic idea of bigger pays more still predominately exists.

Likewise, the pay difference for Capt, F/O, and flight engineer go back to the early days. The original formula was that F/O's and engineers made a percentage of the Captain's pay for a Capt with the same number of years longevity - topping out at 66% for F/O's and 50% for engineers. Those same percentages are still roughly present in the pay scales for F/O's and engineers today.

There have been experiments with single pay scales (with different scales for F/O's and engineers) - IIRC, TWA went to a single set of pay scales regardless of equipment before they folded. HP has that now - the West pilots have a single set of pay scales that doesn't differentiate between the 737, A319/320, and 757.

As McFlyPHL pointed out, the only real reason for different pay scales on different planes today is productivity - pilot's flying bigger planes are more productive and tradition. A side note about F/A's - they have one pay scale regardless of equipment flown. The only difference is that most airlines pay the A or lead F/A a small amount extra for the added responsibilities that come with the position.

Again, McFlyPHL pointed out the difficulty of judging merit for pilots - they perform their job day in and day out without any direct visibility by their supervisors. During my last 5 years, I probably say my chief pilot twice and one of those times was when he gave me my little "Congratulations on your retirement" certificate and the other was when I stopped by his office with a question. During that same time, I probably had a check ride (either an instructor pilot or FAA inspector rode jumpseat) 10 times. Throw in two simulator sessions a year and you can see that there's very little direct supervision on which to base an opinion of merit.

Plus, pilots for major airlines fall with a pretty narrow performance band at the top of the heap merely by virtue that every airline that's hiring has hundreds of applications for every position to be filled so can afford to take only the best applicants. Finally, throw in the typical probation year when a pilot can be let go for just about anything and only the best of the best are retained.

Jim