Why so damn big? - Boxbasher
Why is this car so big? Packaging seems poor, but silly me, is it so that they've got a platform for say a new Saab 9-5?
Why so damn big? - Altea Ego
Thats the trend. Look at the mondeo - HUGE great barge
Why so damn big? - jase1
Think the Saab comment mightn't be so far from the truth with these German-sourced US cars.

They're probably looking to ship them over to the USA when they get round to closing all the American factories, and need the cars to be a bit bigger to suit US tastes.

Either that, or its so GM has a platform for the Daewoo-sourced cars that they'll sell to the US for the same reason.
Why so damn big? - Victorbox
The outgoing Vectra is pretty massive for a family car - nearly as big as an Omega from the generation before. The Omega was considered a large "luxury" car. Guess you are getting more metal for your money.
Why so damn big? - Bilboman
There's been a constant wave of downsizing and upsizing over the last 20 years or so, probably more to do with marketing and image than anything else. In Vauxhall's case, the Senator was killed off and the next generation Omega took its place. When the Omega was no more, surprise surprise the Vectra grew.
Buyers were convinced that the new car was even bigger, better value, etc. and so the cycle continued. Similar story with Ford (Granada/Scorpio, Sierra, Mondeo), Renault (Safrane, Laguna) and even for smaller cars (the new Polo is larger than the original Golf, so in comes the "Polo sized" Fox...)