As for bathrooms in modern US inner city apartments, I believe it's mainly because many bathrooms allow the apartments to be shared by several flatmates. For the developer, it opens the market to more buyers.
American dwellings are larger than European Union ones for several reasons:
- Most US homes are new: All over Europe, people live in much older housing and apartment stock. Go back to 1940, and the US only had a 130 million population, which typically lived in large families in shared houses. In 1940, the European population (excluding Russia) was already 420 million. Thus, whereas the US population has more than doubled since then, the European population has only increased some 50 percent. This meant construction of far more dwellings in the US in the large 80 years.
- Eastern Europe: Eastern Europe has been much poorer than the US during the last 80 years with people largely living in apartments.
- Space: The US simply has more space per capita. We are 500 million in the European Union (for a few days more) in an area half the size of the US.
- Sprawl: The great US population expansion coincided with the automobile revolution allowing for people to live in suburbs far from city centers, which in turn allowed for larger houses.
- White flight: Europe never had an exodus from city centers comparable to the US.
- Materials: Europeans live in brick and mortar houses or concrete buildings. American houses are made of wood and are cheaper to construct allowing for larger homes.