Fewer rooms means less time spent on cleaning and home maintenance. This has been a huge perk for my family, since it means we spend more time outdoors, doing things we love.
With less to build in a small house, everything gets done quicker. There are fewer materials to store and move around the site, less time is required to plaster, paint, lay tiles, and sometimes drying times are also shortened.
Waiting times are reduced when tradies have smaller tasks because they can more efficiently coordinate with one another.