Thanks for your reply. I do not know the file system on the remote side, but I guess it's not the same. This might be a reason, but now I'm not sure about that, because I found something out:
I checked the today's backup. Local file has a size of 1.76 MB according to the Plesk page. When I download it and check the size with Windows, it's still 1.76 MB. Remote file is 1.49 MB according to Plesk. When I download it and check with Windows and also when I check the size directly on the remote FTP via an FTP software I get 1.77 MB (the fact that both files have a different size is correct, as I found out today in another thread (and the remote file is usually larger)).
What I want to say: I think that this is a bug. The remote file size is not shown correctly. Should I create a bug report?