Most people are concerned with climate change but continue to drive bigger and bigger cars. Some people drive big cars because they feel its safer in an accident. Others want a big car because its more comfortable and can carry more stuff. A select few don't want a big car but are forced to buy one as smaller cars are slowly disappearing from the marketplace.
Since the 90's, the SUVs have grown in size and status in the United States. SUVs are the most prevalent big cars on the road these days. As one car company introduces a new bigger SUV, others have to match them else they might think they will lose market share. Companies will never voluntarily reduce their SUV sizes unless outside forces pressures them to do (like $10 oil).
Only the government can break this prisoner's dilemma stalemate with car companies. An outright government SUV ban would be vastly unpopular with consumers and hard to pull off. Instead, the government should put a size and weight limit on new cars/SUVs. Putting a limit on size and weight will break the cycle of ever increasing car sizes. Consumers who fear for their safety and buy big cars and rest assure nobody else will be driving an even bigger car.
The government can make the SUV sizes of the 90's as the maximum size for new SUVs. The 90's SUV were no less more comfortable than the SUVs nowadays. Without the option to increase the sizes, car companies will have to compete for buyers are other aspects like price, luxury or maybe increased MPG. This limit would truly help with the environment, reduce road damage (lighter cars mean less road damage) and be overall for drivers (people can drive a bigger car than others).