SPL doesn't have to make a damned bit of sense.
What happens, happens.
Logic would say that my truck would kill in db drag because of a tiny cargo area, but guess what, my truck is the WORST db drag vehicle I have ever seen or tested. It completely falls on its face when it's sealed up. I gain 7 db just by opening the door.
My truck is quieter on the dash than a TRUNK CAR, with the same power and equipment. Granted my truck is WAY louder outlaw than the trunk car, but I think that's a different topic all together.
Just because a vehicle has less cabin to pressurize doesn't make it louder. Hell, looks at pandas. They have WAY more space than a crx yet they are 3+ db louder with the same equipment. The same goes for 4 door suv's versus 2 doors. A panda with 2 10's in the cargo area is getting close to some of our streetmax cars with a wall and 4 subs.
It has to do with the acoustics/design of the vehicle itself, not so much the equipment. That's why some vehicles are just naturally louder than others.