In some countries (Spain, France, Germany ...) they are huge center-left parties, while in other countries (Netherlands, UK, Italy ...) they are small, true left parties.
But then maybe it all comes down to the "huge" and "small" bit ... perhaps it's inevitable that a party that grows too much starts adopting a common denominator / consensus policy, that's bound to limit the influence of its members leaning too much to the Left.
It is also important, i think, to remember that the large, centre-left parties (the Labour parties) are social-democratic, while the smaller left-wing parties tend to be socialist. And Liberals is just as confusing a term as socialists :)
You've got the most important point: that parties vying for the center start to look alike. This is a reason why Fortuyn had great succes and one of the reason why the 2007 elections will be interesting: Dutch politics has seldom been more polarized than it is now.
