A larger car is likely going to have worse fuel economy and/or range than a smaller car. If you get the larger car, you take that hit every single time you drive it. If you get the smaller car and a roof box, you take that hit only when you mount the roof box. If that's most of the time, then sure, it probably makes sense to get the larger car. But I don't think that's the GP's use case.

The larger car is likely more expensive to purchase in the first place than the smaller car plus the roof box.