This experience refers to car rental company 'VERORENT' at Dubrovnik airport:
Background: booked rental car well in advance prior to summer season by considering appropriate size for a small family (young children, plenty of luggage etc).
Upon pick up only a smaller version of car was available different to what has been booked despite complaining to the head office in Zagreb. Basically the message 'No larger car available despite early booking - take it or leave it'. Of course no use to us since it was impossible for us to accommodate a small family + belongings in to the offered car for our anticipated one way rental. Given it was high season, there was ad hoc no other car of larger size available and if so very few options to a horrendous price.
Luckily we found a family sized car for the one way rental - about 30% more expensive than our 'early booking' with VERORENT, however still considerable less than with other car rental companies. Thank you BINGO RENT!
My advice: 1) Avoid VERORENT; 2) if booking well in advance regardless of car rental company ensure that the booked car is actually available prior to arrival.