Shared Photos Quality

That will depend on the original size. Photos in Shared albums are limited to 2048 pixels at the longest edge. Larger photos will be scaled down when they are shared. See: Use Shared Albums in Photos - Apple Support

When shared, photos taken with standard point-and-shoot cameras, SLR cameras, or iOS devices have up to 2048 pixels on the long edge. Panoramic photos can be up to 5400 pixels wide. You can share GIFs that are 100MB or smaller. ... Videos can be up to fifteen minutes in length and are delivered at up to 720p resolution.

The photos taken with an iPhone (iOS device) will also be scaled down. For example, my iPhone X is taking the phots with a pixel size of 4032 × 3024 pixels. The iPhone photos would be downsized to 2048 x 1536 pixels, also 3.1 Megapixel instead of 12.