K-S1 is close to what you are talking about
:
Compare camera dimensions side by side
K-S1 is 1mm taller, which is negligible, and 2mm less wide. It is affordable, and contains a lot of new technology that the K-x does not have, and a better sensor (less noise at high ISOs)
Unfortunately, I don't think Pentax will ever go back to 12MP. File size is something that will only get bigger, due to higher MP and higher bitdepth. Yes, it is annoying, as we have to upgrade computers, hard drives just to deal with these large files, but if you shoot raw, you cannot avoid it. Shooting jpeg and reducing image size is the only way.
Or cloud storage, and deleting the photos on your actual drive, but that is something I would personally never want to do. Seems too risky, losing all control, especially since some cloud services apparently analyze data and sell research information to third parties.