While tall and large folks can fit in a smart just fine, there are still some problems created by the small format of the vehicle.
One that we noticed right away was the difficulty of fastening seat belts. One person can do this just fine, but when two try at the same time it starts to get a bit "cozy" near the brake lever. With an armrest installed, it becomes even more interesting.
I have the same problems with reaching the seat belts to fasten them. I have found that it works best by leaving the door open until the belt is fastened. Doing it with the door closed is an exercise in contortionist behavior.
Ditto accessing my wallet while in the car. There is enough room there to sit comfortably, but no more. Trying to twist and turn to access the rear pocket where the wallet is kept takes some effort.
It's as if the engineers with Daimler Benz cut away every bit of "excess" space when designing the car, the better to diminish the size of the final package. So, they left us two more than adequate but minimalist volumes, and little besides. Wrap it in sheet metal and plastic, and there you go.
Similar systems are used in the design of space craft and armored vehicles, both weight (and thus volume) critical vehicles in their own way.