Having lived 21 years in German suburbs for now, I can tell you that it has to do with different urbanisation patterns. Suburbs in Germany are usually independent towns or villages who have grown over time because of the proximity to a big city. In NA it seems that cities are just growing beyond their boundaries, leadig to a continuous urban pattern. In Germany, cities usually don't grow till they are fully urbanized, because of zoning laws which prevent them from building up their entire area.
Another point is that the concept of suburban living is very new in Germany. Until WWII German cities were dense. Spreading out began only in the 1950s. In the early 1960s, German cities usually reached their population peak. That was the time when villages around the cities started gaining people from the city which fled the city life or just wanted to have a rowhome or a flat in a quieter area. Foreigners substituted the people moving out of the cities. Single-family-home construction in the suburbs started in the late 1960s and early 1970, when most German families owned a car. It continued until the late 1990s in new areas surrounding old villages and towns up to 30km away from the center of the core city.
Regarding residential patterns, Germany is a country which is dominated by appartements. In the big cities usually 80-85% of the population lives in appartements. Even in suburbs, still half of the population lives in appartements. Only a minority of maybe 35% lives in free-standing one-family-units. In the inner suburbs, two thirds live in appartements. So the concept of a suburb is a different one. You don't move to a suburb to have your own house, but to combine advantages of rural areas and cities. That's why German metropolitan areas are not a contiguous built-up area, like it is in the USA.
