On a Windows 10 computer, I have dragged an excel file from Outlook into a directory displayed in Directory Opus Ver 12 (paid). An older version of the file already exists in the folder, and is being overwritten. The Dialog Box that pops up shows the "New File" as having 0 Bytes, although it is a good file.
Outlook is probably providing the source of the drag & drop in a way that it looks like a zero byte file until the copy actually starts. There probably isn't anything we could do except hide the file size (but sometimes it really is zero).
Does File Explorer show a different size for the file when it asks for overwrite confirmation, if you do the same thing using it?
You're right - in the Windows "File Explorer", the size of the new file is not shown, only the existing file size in the option to leave it and not over-write it.