This is happening to plenty of people, see: https://discussions.apple.com/thread/252110183?login=true&page=1
I submitted a bug to Apple on 11/26/2020.
I have 12 Pro Max, iOS 14.2/14.3 (it happens with both), and Windows 10. The full-resolution photos exist on iCloud.com, but iCloud for Windows app downloads a lower-resolution version.
There are a lot of settings controlling images and cloud, but if it's just because iCloud is downloading it incorrectly, there is a manual workaround. It's a pain, but it will work until hopefully there is a fix. See this link https://support.apple.com/en-us/HT209454 and search for "Click and hold the download button" and follow instructions.