This is quite normal and you would have the same issue as a paying customer. Car hire companies tend not to have larger SUV type cars sitting around and they move them around between branches when required. This can’t always be the same day.
The last time we needed a hire car through Motability I explained that we needed a decent size SUV to accommodate the wheelchair and to ease access, and I was rather surprised to see a Mercedes GLB arrive. Unfortunately it was a Friday built one and I was glad to see the back of it, but the thought was there 😂