Your Pentax 645z images are around 53 million pixels, about half the size of your GFX 100s images, which will make a huge difference in what you have to work with.
This may help or make it worse. You wrote about the file size differences you were seeing in Photoshop as compared to the raw file size, which presumably is the size on your sd card or hard drive.
At the risk of making things way worse, it does get confusing, hard drive size is not the same as memory size even though they have the same units, (MB).
I think, based on what you are writing, the results you are seeing are correct.
All that is true, awesome images, but everything says that the uncompressed files are around 200 MB, it seems like something must be wrong to be getting nearly 600 MB. My old Pentax 645Z was 50 MP and gave me 100 MB files. If this is correct then I’ll work with it
That camera puts out 102 million pixels per image, at 16 bits per pixel. An image is going to take a very large chunk of computer memory just to hold it, then when you have image processors adding in their parts and storing that on a hard drive, the file size will be, well, …, also large. You really need good file management practices to handle those files, but the beautiful images you can squeeze out of that sensor are remarkable.
Even though you store the raw file as compressed (by the way, most place say the compressed lossless file size are around 100MB, and the compressed lossy file sizes are smaller) the image processor has to uncompress it to work on it, and any further storing will rely on the image processor’s file saving, not the original raw file.
Shorter: you need lots of hard drive space and computer memory to work on these files.