I thought the mavic shoots 12 mp photos. My photo files are around 5 mp in 16:9 and 4 mp in 4:3.
Shooting in raw boosts to 17 mp and then after color grading and exporting as jpeg, it goes back to around 5 mp.
Is there a setting in the DJI GO 4 App to maximize the resolution of the still camera?  

How do you get a 17MP raw file?

Are you  sure that you do not mix up file size (MB) with resolution (mpx)? The camera is able to capture 12 mpx (12 million pixels) no matter if the file is saved as RAW or JPG. If I recall correctly, 16:9 has a few pixels less than 4:3 as the camera is 4:3 and to get 16:9 a few pixels are cut off.