So I just bought UT 2004 ECE to have a DRM free copy of the game, as I already have it on disc. And checking it on my account page I found it to be rather....small.

The box comes with 2 DVDs, and states the game requires 5.5gb space, while the installer takes up 2gb space. Has compression really come that far by now?

The back states that DVD 1 came with six new onslaught maps, three new onslaught vehicles (Cicada, SPMA long-range artillery vehicle and Paladin (ground tank)
Six new characters and eleven additional mods created by Unreal players. Is this included in the installer?

DVD 2 seems to be a lot of video tutorials for unreal scripting, mutators, weapon design, maya fundamentals, static meshes, character development and vehicle design. Were these left out of the bonus content because they're likely supplanted by much better online tutorials? Or rights issues? Bandwidth concerns?

The game is indeed there, just curious about the smaller size and lack of bonus features for an edition which was carried mostly by bonus features.

maybe DVD Version multi language !?

but 3.5 GB only language? maybe its really the compression.

Just checked the disc. Could very well be compression
There are 7 folders named CD1 through CD7. And on them seem to be fairly uncompressed data as in the installed directory. They do however seem to be for instance .uz2 files (unreal zip 2?)
Those folders account for 3.7gb, compressed properly that could well be 2gb data
In the root there's also a 1.6gb .cab file which I can't tell what is, but might be related to the UT2004 Editors choice Edition Mod Installer.msi file Maybe those haven't been bundled, maybe they are just better compressed. I should likely just download and install from Gog to test it out.

Still the DVD of video tutorials isn't included. Could be superceded by Youtube or other places. But if the Gog team has issues sourcing the data I'll happily provide data for them

Yes, the compression really come that far by now. The files used by the Unreal Engine can be compressed with high ratio. The game is about 5,5 Gb when installed, if you try to compress it with high-catch rar, or 7zip, it will be even smaller than the GOG installer. However the GOG installer install it faster than extracting from archive.

OP is correct, the eleven extra mods seem to be missing from the game. See "Editor's Choice":- http://liandri.beyondunreal.com/Unreal_Tournament_2004#Bonus_Content

What we actually get is the base game plus the mega pack (extra maps).

I bought Unreal Tournament GOTY on Steam last week and all the GOTY content was missing from that as well. I know you can download mods individually yourself but these games are being titled dishonestly somewhat.