Sparse file
A sparse file has long empty stretches that the disk doesn't actually store, so it takes less space than its size suggests. Folder moves keep it sparse.
A sparse file is a file with long stretches of nothing in it that the disk doesn't actually store. Its size might say tens of gigabytes while it really uses far less, because only the parts with data take up room. Virtual machine disks are a common example.
The risk comes when copying: a copy that writes out every empty stretch can make the file use its full size on the destination, filling the drive faster than you'd expect.
When Storage Studio moves a folder between your Mac and your external drive, it writes the empty stretches back as empty stretches, so a sparse file uses about the same space on the drive as it did on your Mac.
See also: What a move keeps · Hard link