Unexpectedly, SIGMA Corporation has introduced the world's smallest and lightest full-frame mirrorless camera. The SIGMA fp is extremely modular, weatherproof and comes with some very interesting features.
https://www.notebookcheck.net/SIGMA-fp-The-world-s-smallest-and-lightest-full-frame-mirrorless-digital-camera.427415.0.html
No flip screen, or else this would be the perfect backup camera paired with M mount lenses. The size passes as a compact itself.