Are you viewing these photos in the Windows Photos app, OneDrive online, or another program? Also, does the file size change when the photo looks smaller?
This usually happens when the photo is being displayed with the wrong zoom, aspect ratio, or resolution, or when the viewing app falls back to a preview instead of the original file. If you are using the Windows Photos app, open the photo and select Zoom to fit or Actual size. You can also try resetting the Photos app settings, which often fixes display glitches. https://support.microsoft.com/en-us/windows/get-help-with-microsoft-photos-app-702104c6-7541-6757-d624-c0846b3bdbd3.
If these photos are stored in OneDrive, make sure you are opening the original file and not a thumbnail or preview. Sync issues can cause OneDrive to show a low resolution version with black borders until it fully downloads. https://support.microsoft.com/en-us/office/fix-onedrive-sync-problems-0899b115-05f7-45ec-95b2-e4cc8c4670b2.