"What do you get in the place for this lower efficiency? Are American cars that much heavier? More comfortable? Faster?"
I cannot speak for everyone, but as someone who lives in the US and also spends a lot of time in Europe, there is a very big lifestyle difference at play here.
First, I have a five person family (wife and three kids) and you[1], statistically, as a European, have 1.6 children - and as a highly educated HN reader, probably less (on average).
Second, Americans move around a lot more than most europeans do. On the occasions that I have lived in Europe, I always spoke of how interesting it was that we could just "drive to St. Petersburg" or "just drive to Transylvania" or whatever, and otherwise smart and interesting european friends would look at me like I was a crazy person. It is very, very rare to meet Europeans who drive across multiple countries for leisure.[2]
So when I drive five people, weighing over 500 lbs total and accompanying luggage the equivalent of one Europe away and over two mountain ranges[3] I am going to need something more than a lawnmower engine. This is not an edge-case - this is something we do multiple times per year.
That being said, there are some very interesting cultural differences in the other direction ... once while in Denmark I saw a decent sized horse trailer being hauled by a nice, new Audi A8. Basically 100% of the driving public in the US would have considered this a gaping tear in the fabric of reality.
[1] Not you, but you on average.
[2] Zurich to Transylvania is 1800km - about the same as SF to Denver which is ~2000 km.
[3] CA to MN, for instance, over the Sierras and then the Rockies.