The BMW X3 is a compact crossover SUV, meaning it sits on a car platform—specifically, the BMW 3-Series’s platform. A true compact SUV sits on a truck platform**, but if you’re just considering the interior size and the exterior length and width, the BMW X3 would almost certainly qualify as a compact SUV.
The BMW X3 has seating for five and up to 62.7 cubic feet of cargo space with the rear seats folded or 28.7 cubic feet with the rear seats up. If you wanted to go any smaller, you could always opt for the smaller BMW X1, which is a BMW subcompact crossover SUV that seats five and has 57.2 cubic feet of total rear space, including 25.7 cubic feet behind the second-row seats.