I suspect part of it is that the housing being built now is both bigger and better than that built in the 60s and 70s. Think of what cars were like then versus now. The other might be the availability of land. Another factor is how housing has changed from being more of a commodity item back then (its a place to live) to an investment vehicle (its a thing you own and make return from). These trends are not specific to Germany, but would apply to many developed countries worldwide.