Why? If you have a large object, lots of luggage, or four people in your party, fair enough, but if you are cancelling just to get a bigger (and presumably better) car, then this is quite selfish. It wastes your time and that of the driver.
If I get a ping, I start driving immediately. I usually don't work at times when I have to pay for parking, but when I do, it might mean I've lost a parking spot and have to hunt for another. I'll tell myself, these things happen, but it's not pleasant.
I drive a Golf - the wagon version, because I found the hatch was often a bit tight on airport runs - and it's plenty big enough. I might have to move the front seats back and forth a bit to distribute legroom, but it's okay. I can fit pushbikes and things in, and it's remarkably roomy with a bit of juggling and VW's clever design features.
But I get the odd cancellation. I guess that this is why.
Smaller cars are perhaps associated with drivers who are less well off. I guess if a passenger doesn't want to mix with a driver who's a bit further down on the bank balance scale, then that's their privilege. But Uberdrivering isn't a handsomely-paid job, and sometimes a driver can't afford a big car. Cancelling them just makes their life even harder, and it won't work to drive small cars out of the Uber fleet, because so long as a car has four doors, is roadworthy, and is less than nine years old, it will be accepted as an Uber. There's no policy on size.