You can at least partially trade width with depth, that is, making a speaker narrower but design it so that the cone can travel farther and move the same air volume, but it also have drawbacks, and thicker speakers are also harder to accomodate.

The smaller the volume of air you need to move, the less this matters (which is why IEMs have more bass than tiny bookshelf speakers, despite being way smaller. It should be theoretically possible to do just replace the trunk and backseat of a saloon and have a 2 seater with good bass extension

I think we are taking about the bass shaking your chest. IEMs can’t do that.

IEMs can develop high pressures at low frequencies because the volume of air they are moving is very small. A compact car with the windows rolled up here is analagous to the ear canal in the IEM. If 50% of the internal space of the car is speakers (easily achievable if you remove the backseats and use the trunk space), then you ought to be able to generate large pressures at low frequencies as long as the windows are rolled up.

I'm not sure that's entirely true/necessary. Standing waves in the cabin can help compensate a decent bit. A good transmission line speaker can likely fit in the boot/trunk, though stereo imaging will likely be poor in any car audio system I think.