I've stayed in this house http://i.imgur.com/EgZRY9g.jpg It easily held 30 people. There was a large upstairs room with many beds side-by-side, and 5 other small rooms with one or two bunk beds each. This left the downstairs mostly open with a big kitchen, two medium-sized meeting/dining rooms, and a big classroom.
Well for starters, the surface area per person is a lot lower. Even with framing and siding, it's probably less material per person than building a little bubble around each person. Certainly running 20 little air conditioners is less efficient than central air. Maintenance over time is probably lower. The big deal is that it uses way less land per person, partly by using a two-story design which a dome can't do.
