A few weeks ago, we had a full size Emerald Reserve rental. When we asked an attendant about more options, he said to go ahead and pick a car off the Emerald Aisle if we wished.......... so we took the Aztec, lots of luggage space in that vehicle! (the full size rental was only $4 more than Emerald Aisle, so I did not care about that).
So if you see something better on the Emerald Aisle, and you booked full size Emerald Reserve, you can probably take a vehicle off Emerald Aisle.