Why do .old files show different sizes in Windows 10 systems?

That's normal and done out of performance considerations. You can right click > Properties for each folder to see their sizes or use tools like TreeSize.

Sometimes, the .old folder doesn't contain a full snapshot of your previous system, especially if the upgrade or backup process was interrupted.

The Windows.old folder can show inconsistent sizes across different tools because of how they calculate and display disk usage, especially with system and hidden files. To determine its actual disk usage.