People charge, in general, more money to detail larger vehicles. If someone asked you for a discount on a mini-cooper or other similar small car, would you do it?? How much??
Discounts are reserved for when you screw up, not small vehicles.
You should charge according to what you understand the owners' expectations are for the finished product. You should also have an understanding of your hourly operating cost. Take how many hours it will take for the finished product and multiply it by the rough amount you need to make per hour to walk on water.
If I polished the exterior of a Hummer in three hours or sped through a full interior/clay/seal in three on a Mini, they are both going to pay the same. . . probably
I just charge them for how long it is going to take. Since smaller cars usually take less total time and product, I don't charge them as much as I do larger cars.
Like Scottwax said, it is a good idea to factor the size of the vehicle into your price. Then you would be able to show the client that the size of the vehicle has already been taken into account and no further discount is needed.