Google Photos has amazing software for search and organization, but it compresses your photos to 16 megapixels.
This is a setting you can configure for yourself. You can either back up your photos in "high quality" (16MP) for free, or you can back them up in "original quality" using your storage space:
Every Google account has 15GB of storage space for free. This space is shared between Photos, Drive, Gmail and probably some other apps. If you run out of space, there's a range of plans starting from 1.99/month for 100GB: