I don't think I'm making too many assumptions when I say RAW photos from a DSLR being uploaded to Google Photos is not anywhere near the intended use-case.

However I still agree that 15GB of photos is pretty close to nothing these days.

Well... their own marketing does say:

"A home for all your photos and videos"

and they have a blog post titled:

"Easy Wi-Fi backup from your Canon camera to Google Photos"

in which they say

"you can choose to automatically transfer original quality photos to Google Photos"

Interesting, I didn't know there was a camera that could do it directly. I always did want that for my Sony a6000. But for jpeg, not raw. After sitting in front of Lightroom for hours upon tedious hours adjusting vacation pics, I will never shoot in raw again.

You should use compression to not use so much space.

These photos are compressed.

The uncompressed raw files (or TIFF) are much bigger.