I'm not a car expert by any means, but I just did a quick look through the Euro NCAP database, and apparently there are some models with 5-star ratings that are

I'm not a car expert by any means, but I just did a quick look through the Euro NCAP database, and apparently there are some models with 5-star ratings that are below 1.5 metric tons. I don't disagree that safety features don't incur additional weight, but it stands to reason that not all the weight gains are caused by additional safety features. It appears to me that it's more about perceived safety, which leads to an arms race: If everyone around you is driving a 3-ton SUV, you'd be less inclined to choose a Smart ForTwo, even if it's sufficient for your mobility needs and more economical, because you fear what happens when one of the SUV monsters crashes into your car. There are some ways to fix this: banning SUVs (e.g. by capping vehicle weight for individually-owned cars), providing better public transit options, disincentivizing car ownership in urban areas (e.g. by reducing parking spaces), etc. Obviously none of those policy proposals are going to be popular.