Oh, the trehuggers have a comeback to that one too...
I've heard two versons:
1. "yeah, but what if all drivers would get bigger and bigger cars to protect themselves? then we'd all end up driving semi trucks" --> my answer "oh, boy would I love to be allowed to drive a semi to work"
2. "yeah, but what if it's you that makes a mistake in traffic and you crash into a smaller car and kill the driver and his family? There's less risk of killing them if you're in a car of similar size" --> my answer: "that is indeed a risk, but from what I see on the roads, I think chances are much higher that someone else crashes into my car than I crash into theirs. I drive very defensively and am very careful, and I never push the truck to its limits, because I know it's not as nimble as my Civic and I know that any mistake I make could kill someone much easier".
and ultimately, my answer to their why is... "Because I f'kin LOVE trucks!"