Meh, I'm currently on a cross-island road trip with. Family of four in a >10 year old, manual, brown diesel grandtourer/station wagon. Trunk and roof box fully loaded.

Long range fuel consumption at 5.3L/100kms, as usual at summer conditions.

Of course, ymmv : - )

Sure, but I only used the roof box on occasional long trips when we had lots of luggage (e.g., visiting the grandparents interstate for Christmas). So I might have paid a few tens of dollars extra in fuel, but it was far certainly cheaper than buying a bigger car when we otherwise didn't need the extra space.

The roof rack and Thule box cost maybe $1200 AUD. But the rack was also useful for attaching bicycle carriers.

Needs to be done in comparison with a larger car that would fit the same cargo as the smaller car with a roof box.

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.