[buran77]

It's a safe assumption that safety requirements were a big driver of increased size and weight. And some of this factors have a snowball effect. Some requirements make the car bigger and heavier so it needs bigger engine, wider tires, bigger fuel tank, then it's even bigger and heavier.

Add all the luxury elements that attract customers and you have a lot more extra weight. Small cars are generally seen as entry level, unsafe, so car manufactures are more than willing to provide bigger, fancier models. It's not just the size that shot up, prices did too in order to cover all the extras.

People are also bigger and heavier than they used to so extra space and carrying capacity are more than welcome.

[bluetomcat]

In Europe in particular, there was the diesel craze of the 2000s. Few cars in the 1990s had diesel engines. The average family car was something like a Ford Mondeo with a 1.8 NA petrol engine.

By around 2005, almost any "premium" mid-sized car like a Mercedes E-class or a BMW 5 series had a turbocharged 6-cylinder diesel engine around 3 litres with common rail injection. These engines have much heavier blocks and need a lot of additional systems for emission control.

[jmrm]

I think this could be different depending of the country. In Spain the most popular engines in premium cars in the 90s were the 2.0L and the 2.5L turbo Diesel, and lots of compacts and mid-sized sedans, specially at the late 90s, were sold with 1.8L to 2.2L engines (Peugeot 205, Citroën Xsara, Renault Clio, Opel Corsa).

At the early 2000s, about 90% of the cars sold were Diesel IIRC. We always care a lot in terms of economy where buying a car, and with a Diesel oil cheaper than gas, and Diesel cars that had more mileage than petrol ones, there's the reason.