When I turned on the GOG Downloader the other day, I found Spellforce 2 DoP in the queue. I thought that was strange, because I should have already had it from December, but I let it go. Long story short, once it was done, I found that it was the version I already had (GOG version 2.0.0.4) except the new installation had 4 .bin files, while the old version had 5 .bin files. The first four files are the exact same size in the new and old versions, but the mystery 5th file is around 575 MB.

Any idea what on earth I have here?

It must be a glitch, as the current version consists of 1 EXE and 5 BINs, where the last one is about the size you mentioned (while the German version is 100 MB bigger). All the other Spellforce titles are way smaller than Spellforce 2 DotP.

Very strange. Thanks to you both!

Indeed strange. It's not even clear if it's due to false server info or a glitch of the client. Hopefully it's a one time issue. Please keep us posted if not.