Hi dingerk,
Welcome to the Apple (user-to-user) Community.
Re: The same photo downloaded from iCloud is 1,143 KB (JPEG)
Wondering if your PC is able to accept HEIF or HEVC media
Info. on Using HEIF or HEVC media on Apple devices - Apple Support
Eg: says: "Importing this media via USB:
When you import HEIF or HEVC media from an attached iOS device to Photos, Image Capture, or a PC, the media might be converted to JPEG or H.264.
You can change this import behavior in iOS 11 or later. Go to Settings > Photos. In the TRANSFER TO MAC OR PC section, tap Keep Originals to prevent the media from being converted to JPEG or H.264 when importing".
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in Apple Support's article:
Download iCloud photos and videos - Apple Support
says:
"If you're using a PC, you can set up iCloud for Windows and download a copy of your photos and videos from iCloud Photos".
All the best :-)