The airline industry, for the most part, is a seniority based system. You start at the bottom, usually in the lowest paying seat, in the lowest paying aircraft, then as your seniority improves, you get to bid to either a new seat, or a new aircraft, or both. The usual transition is (considering no major airlines have flight engineer seats anymore) you'd start as a first officer on a narrow-body jet, then move up to a larger jet (either narrow or wide-body) depending on your seniority and the openings in those seats. You might do this two or three times before you are finally senior enough to hold a captain position, again, usually on the smallest jet your company operates. Then it all starts again, with you waiting until your seniority and seats available allow you to hold a captain's seat on a larger jet. Then the same thing again.

Of course, there are some guys who will make captain, only to down-bid to an f/o seat, usually for quality of life reasons. Most guys who make captain don't want to give up that authority again, so they either stay in their seat, or wait until their seniority can offer them another captain's seat in a larger jet.

As has been said, pay scales start at the bottom on the smallest jet, and go to the top on the largest jet. This is true at the vast majority of airlines. Some guys chase the bucks, others chase quality of life, and others try to find a happy balance between the two. If I were to do it again, I'd marry a independently wealthy woman, then do the flying that I enjoyed the most, regardless of what it paid. But that's just me.

JJ