What.The.Fuck?

I'm not sure whether that's the culprit or not. The linux versions on the main site (that is, the full downloads) aren't even close to that size, and are actually much closer to Windows's size so it'd be closer to half that if you included both 32 and 64 bit. If you're talking plus DLC that does add heft, but it's the exact same file for all versions so it doesn't matter here.

Score another one for outside of GOG I guess.

I just downloaded the 4.1 GB package of »Tales of Maj’Eyal« and installed it to satisfy my curiosity about this issue. Well, the reason for the size factor of about 10 is that the GOG package indeed contains the game ten times – five times for 32-bit systems, five times for 64-bit systems. The versions within this installer are 1.6.1 as well as 1.6.4 to 1.6.7. Hopefully this will be cleaned up soon.

Two questions remain: Was that huge package created intentionally and was it GOG or the developer who (mistakenly) inflated the installer?

Well I just asked DarkGod and got this reply: I think GOG packages their Linux installers manually, which is why they usually take longer to update and sometimes contain little blunders.

EDIT: I sent a ticket to support.

I also sent a ticket to support, and whatmore, I got a reply that it is being investigated and corrected.