Cannot download high quality video files using icloud shared links

I have my high quality videos recorded by my iPhone automatically synced to iCloud. On icloud.com, after I sign in on my Mac, I can download my videos with sizes close to the sizes recorded in my iPhone's video information with the Unmodified Originals option. However, if I create an iCloud share link, even if I choose the Unmodified Originals option using the link, the file size downloaded is only about 1/20 of my original in my iPhone. Why is that so and how can I fix this problem?