A lot of time it depends on what they have around the lot, as well as your status (5 Star or President's Circle), how many rentals you have for the calendar year, and what kind of rate you are paying.

As a 5 Star, I normally received two to four class upgrades when renting below full-size at SEA. When travelling, I normally rent Premium so that is what I get. My HLE will give me a full-size or Premium on an Economy rental, but I have a great relationship with them.

If a location has an abundance of one type of car, they often assign them. I've rented Compact and received Mustangs or mid-size and received Tauruses with NL.

As President's Circle, I am guaranteed a one-class upgrade (maximum full-size to Premium) and so far, my first PC rental at SEA earned me a YG (Specialty Premium) on a full-size rental. I often rent for one or two days on a weekend, so usually that gets me something nicer since large airport locations have plenty of cars over the weekend.