Why don’t rental car prices always run in order from the smallest cars being the least expensive to the largest and fanciest being the most expensive?

It’s all a matter of supply and demand. Compacts and mid-size cars are the most popular, so while vendors have the most of these they also sell out the fastest. If a vendor is low on them or anticipates being low on them based on the advance reservations, but has plenty of specialty vehicles or SUVs, etc., they will lower the price on what they do have to encourage renters to use those vehicles and allow the vendor to keep or get back up to a comfortable available inventory with the smaller vehicles as they are returned. This inventory balancing act is an ongoing process beginning weeks in advance as the vendor looks at their projected fleet and the number of current and anticipated reservations for a particular time period and continuing right on through to real time.