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.