Australians love 4 wheels drive and bigger cars. In some cases it's justified for obvious reasons: rough environments with less infrastructure (bush, outback..). In other cases it is for softer reasons: a big camping culture, having a big car being a social status, towing your boat/jet ski etc.
Also a few other points to consider:
_ Australia enjoyed economic growth for a long time and Australians are rich.
_ Fuel is cheap. According to www.globalpetrolprices.com right now a liter of gasoline is 0.74 euros compared to 1.33 for France
_ The road infrastructure is more favourable for big cars than in Europe (big/plenty parking spots in most towns in Australia).