That is fantastic to know about size and makes so much sense. I was looking up photos of some American show dogs and then working Euro lines and kept thinking some of the Americans looked so slight in comparison! If they are immature that would make a world of sense.
Are you sure you were looking at working line dogs and not show line dogs? As Meadowcat said, the standards are similar (differing by about an inch in height at the max), but a lot of European show line dogs are not only oversized, they are shown several pounds over their ideal weight.