There are different levels of "source code" when it comes to a video game.
There is the actual programming the makes the core of the game controlling logic, input, AI, etc. which is usually quite small a few dozen MB usually.
Then there is varying amounts of things like scripts for compiling data into a format the game understands, the raw data for assets in the game that can be used to modify assets in a useful way before importing them into the game itself, utilities including things like a world editor (which will have its own source code as well as being used to create data for the game).
The full "source" of a game will have all the files needed to build the installation of the game from scratch including the original raw assets, etc. which can include things like much higher resolution textures and higher detail meshes, etc. than the version shipped with the game.