The sync settings for Google Photos let you select two different quality options.
"High quality" which reduces resolution and file size, but uses unlimited free storage not counting toward your Google Drive quota.
"Original" which retains original resolution and file size, but uses up your Google Drive quota so is not unlimited.
Details about what Google actually does with RAW images is light, but at least in Google Photos' former incarnation, RAW images were converted to JPEG upon syncing.
Now, I'm not confident this has anything to do with your problem, because even with the former option your photos should still be saved at up to 2048x2048 pixels. That said, DNG might be different.
Check this setting and see if setting it to "Original" fixes it.
I just re-read the question and it looks like you copy the file manually to your Google Drive rather than letting Google Photos do the syncing and storing itself. It's possible that in this case, Google Photos has limited RAW support and is only viewing an embedded JPEG from the file.
Try syncing some of the DNGs using the Google Photos sync feature and setting the sync quality to "Original" and see if this fixes it. Even though the final result will still end up on your Google Drive, the fact that it's a photo that went through Google Photos may make a difference.