Get Canon 5DS DSLR at B&H Photo.
Presented in my review of the Canon 5DS R, I show that mRAW is more complex than simply a crop and a downsampling of full-resolution raw.
The analysis includes the full-res image downsampled to match mRAW resolution, and compares it to the mRAW in two ways as well as three large 3-way crops (RAW, mRAW, mRAW with extra sharpening).
There are other differences that one should be aware of. At this time, my recommendation is to avoid mRAW entirely. sRAW looks more promising but off to the fireworks.
How mRAW is done by the Canon 5DS / 5DS R
To make an mRAW or sRAW image, in essence the Canon 5DS / 5DS R crops the 8688 X 5792 image size to 8640 X 5760, which is exactly 4/3X the dimensions of mRAW and 2X the dimensions of sRAW. The scaling from full resolution is thus 3/4 and 1/2 to get to the two smaller raw sizes (after the minor crop), and thus the mRAW and sRAW formats are slightly cropped from full frame.
RAW: 8688 X 5792 ==> 8640 X 5760
MRAW: 6480 X 4320
sRAW: 4320 X 2880