Yes, it is, especially when you consider utilities. Average house in the burbs has some lawn or shrubs or something that uses water, and it's not like well water is ultimately 'free' - it comes from somewhere. Also, it means less land developed, less extensive electrical and water distribution systems, and so on.

Yes. Cities, dense cities, are efficiency machines.

Look up Geoffrey West, of the Santa Fe Institute, who does all kinds of research on scale and scale effects. Including of cities.