It will be the size of the car seats on the market. In the US people have bigger cars (in general). Parents seem to want to maximise the size of the seat. As

It will be the size of the car seats on the market. In the US people have bigger cars (in general). Parents seem to want to maximise the size of the seat. As a result, it's my hypothesis that small sized car seats do not sell well. The flow-on from this is that it probably would be very difficult to purchase two car seats that would fit comfortably into a small vehicle, like a VW Polo or similar. In addition, there are other factors that drive the choice of a larger car, including space for the various other things that babies come with, like prams. And most people prefer larger than smaller when choosing a vehicle. People in Europe choose smaller cars for a variety of reasons, including parking, fuel costs and road size.