Large cars tend to consume more fuel. They're also usually more difficult to maneuver, particularly in cities that have narrow roads or limited parking. Mid-sized cars tend to be the most popular rental vehicles in the United States. Most have better fuel economy than larger cars, and they tend to be less expensive to rent. You may even find that when you reserve a compact or subcompact car, you'll often receive a free upgrade to the mid-size category. That's because most rental agencies only keep a few smaller cars in their fleets.
Remember, of course, that you aren't guaranteed an upgrade -- you may get exactly what you asked for. If that makes you nervous, try to choose a car that's large enough for you. Avoid upgrades to big SUVs or other large cars unless you have a real need for storage space. But there's nothing wrong with renting an SUV if that really suits your needs. Large families with lots of luggage should pick a vehicle that will provide a safe and comfortable ride.