People want square footage and comfort, not design. Frank Lloyd Wright homes look stunning in an Architectural Digest spread, but living in them is not really up to par for modern standards.
The main house uses 9,300 square feet of which 4,400 is outdoor terraces, while the guest house totals 4,990 square feet of which 1,950 square feet is outdoor terraces.
https://fallingwater.org/media-resources/fallingwater-facts/
The newest homes that FLW had a hand in building date from 1959.
By the standards of the time, they were comfortable (if a bit lacking in closet space).
If you'd like, you can buy a modernized kit Usonian (inspired by the Jacobs I house) from Lindal here:
https://lindal.com/home-designs/madison/