APFS or Apple File System was introduced in 2017 in macOS High Sierra. Some of its most important features aim to reduce the disk space used by files, such as ‘clones’. There are some considerations to keep in mind, when analyzing the disk space:
First, when a file is copied on the same Volume its contents are not duplicated, though those files can be considered as duplicates by Purple Tree.
Now, even if this file is modified, the unmodified parts of it are not duplicated. Therefore the file sizes are merely an indication. The sum of every file sizes can actually be higher than the Volume size itself!
Read more in this article on Apple Developer site.