I do not believe a reservation is a guarantee for a car. It is possible they could run out of vehicles for any number of reasons (low returns and high walk-in pick ups, time of day, etc). Usually when this occurs, the dealer offers an upgrade of some sort, but not sure what upgrade you may get if it is a full sized SUV.
This likely won't be a problem, and they can fall back on their affiliate (
Avis) if a car is not available and you stick to your guns.
However if a high value client walks in without a reservation, late at night, with only one SUV available, and the client wants it, guess what you get...
I would not worry, but it is not guaranteed, even if you paid for it in advance. if you are concerned (should not be), then call
Budget and ask their policy.