Hey Sam, how do upgrades work at airlines? As you move up the seniority list do you move into a larger aircraft? Do you move back into a smaller one when you upgrade to captain?There are two answers to this question. There is the way it works at most airlines, and there's the way it works at my airline.
At most airlines, both First Officers and Captains are free to bid for any opening that might exist. Whether they are awarded the slot depends on whether somebody senior to them also bid for it. In most cases, you are not required to bid for anything. A first officer could remain a first officer on the smallest equipment their whole career if they chose to do so, although they'd be giving up a lot of pay. A less extreme and more common example is first officers not bidding for a captain's slot until they know their seniority would let them hold a decent line. For many people, not having to sit on reserve or fly crappy trips is worth the lower pay.
The only problem with this system is that filling one vacancy creates many others - using United as an example, a retiring B777 captain's slot might be filled by a B767 captain, whose slot might be filled by a B737 captain, and so on down the line with B777, B767, and B737 first officers. This leaves a B737 FO slot vacant, so you also have to hire somebody new (or in United's case, recall a furloughee). So we're talking about training six different pilots to replace a single retiring B777 captain.
Pilot training can be very expensive, so airlines are understandably eager to cut down on pilots jumping around. Most airlines have seat locks, which mean that once you move to a new position you must wait a certain period of time (one or two years) before bidding another position. At my airline, the company and union signed a side letter which essentially seat locks all first officers. Your equipment is chosen when you're hired, and you stay there until you upgrade. Of course, this would arbitrarily give some FOs higher pay than others, so the side letter provided for pay by seniority. If Miniwhackers comprise 35% of the fleet, then the FOs on the bottom 35% of the list make Miniwhacker pay regardless of what they actually fly. I'm currently on Megawhacker pay and have about 70 numbers to go before I make BarbieJet pay.
Miniwhacker first officers in particular dislike our airline's system. Even though their pay isn't affected, their quality of life very much is. Miniwhacker trips can be brutal: four days of 7-8 legs per day. FOs on the Miniwhacker would like to have the option to bid for BarbieJet slots, where they'd often fly 2-3 legs per day.
Many first officers at my airline have chosen to delay their upgrade until they can hold a Megawhacker captain slot. I personally will take the very first slot available. Before, I assumed that this would be in the Miniwhacker, but now that they'll all be gone in a few years it will probably involve just sliding to the left seat on the Megawhackers I fly now. I doubt I will ever touch a jet at my current airline - right now it takes about 14 years to hold a BarbieJet captain position in Portland, close to 20 years to hold a decent line!