In this context, "monster" means two things. 1) The car is big, and 2) the writers disapproves of them. SUVs are big, heavy, cars. "The Economist" thinks they are frivolous and a waste of fuel--burning gasoline to satisfy someone's ego. There is a quite different use: "monster TRUCKS." Use Google Images and search for "monster truck." Here's a good one: http://tinyurl.com/mrgn5h8 These are hard to explain. There are so few of them that they don't matter much in terms of fuel economy or carbon footprint. Building them is a hobby for some people, and they are fun to watch to people organize "monster truck shows" which are somewhat like a circus. For a very long time, gasoline was cheap. In the United States. Then and now many people in the U.S. live in regions that are not densely populated and have good high-speed roads. My wife's parents lived on a farm in a rural area of a midwest state. They were 8 km. from the nearest town, population 3,000; 15 km. from a town of 10,000; and 50 km. from a city of 50,000. You could almost say they NEEDED a roomy, comfortable car. I'm not trying to defend the U.S. love of big cars, but if you visit the interior of the country--Iowa, let's say--it become easy to understand.

Monster cars are big cars. Usually the term is used for cars that people put big wheels on, but in this case they are just referring to SUVs, which are very large cars that carry around 7 people with a big trunk.