Nah, they just thought "Well, they say a crumple zone is a good thing, so let's make ours extend all the way to the rear seat. It'll be safer!"
But seriously, the smart car’s frame being lower than so many other frames is the reason that I would not ever get in such a car. That and the fact that I can fit in it. It’s a clown car, and I am 6’4” 260 lbs WTH are they thinking making something like this ?!
I'm not trying to sell you one, but there are some myths here. You should be able to sit comfortably at your size. The driver compartment is quite roomy, since it's almost the entire enterior space. As far as the frame, it doesn't sit low, but high, one of the safety features.
That's the frame. Sure, it can never be safer than a much larger well-engineered car, but it's not as dangerous as it looks, especially for its size. I saw a driver's side head-on test once, and the Smart dirver would have walked away. The driver of the other (more normal size) small car would have been in the hospital.
One example, in a frontal, the rear-end takes damage. It routes the energy around the driver, to include using the tires as part of the crash structure (hey, that's a big rubber donut there you can use to absorb energy).