Question: Sharing Full Resolution Photos Using iCloud
There is a locked discussion that discusses the fact that Shared Albums via iCloud Photo Sharing down-samples the original photo's resolution (https://discussions.apple.com/thread/7041212), and asks why Apple made that decision.
I had the just encountered the same limitation and frustration. I'm running Windows 10 and using iCloud for Windows, and the Shared Albums are synced with a dedicated folder in the Pictures special folder. Even those images are down-sampled.
I think I found a solution that, believe it or not, Apple has provided, but it's not well publicized. I did a search in Apple Support for "sharing photos from iCloud," and almost every result discussed "Shared Albums". While this is a convenient method of sharing, the full resolution photos are not available. For some strange reason, Apple doesn't publicize a method I just stumbled upon that works flawlessly, whether initiated from my phone (6S, iOS v. 12.4.1), or from iCloud.com: using a copied iCloud sharing link to then send via email or text to whomever we like gives them access to the full resolution version of the photos!
In the "All Photos" album on the iPhone (not any Shared Album listed lower down the screen, which only has down-sampled images), or the "Photos" Library at the top of the left panel in iCloud, select the photo or range of photos you want to share. On the iPhone, tap the action box (with the up arrow), and, on the lower row of options, select "Copy iCloud Link"option, and paste it into an email or SMS. On iCloud.com, similarly select photos, click the share or action box on the top right border of the window, and choose "Copy Link".
There are email options in both locations, but most mail servers have size limits for attachments; the links get around this. Voila - the recipient gets the full resolution photos, all pre-selected, with a prominent "Download" button which downloads a zip file to the destination of your choice.
I selected the same 14 photos in both locations (phone and iCloud.com), and it worked identically. The 14 photos came in with a combined size of 26 MB, or an average of 1.86 MB each! Finally, as a test, I directly downloaded one image from iCloud.com, and it's size was identical (1.52 MB) with a copy downloaded after opening the iCloud sharing link.
This is full resolution sharing via iCloud!