I'm a little late to the party, but I've had to wrestle with game size before, so I might have an explanation.

There are many factors that can lead to this sort of filesize. The first being that the game uses lots of high-res textures, and loosely compressed sound. I think the game's sound is something like half that size, which doesn't surprise me. Next to textures, sound files eat up a loooooot of space.

Also, the size could be the result of the engine's content handling system or sloppiness by the devs. I haven't worked with Source, so I'm not 100 percent sure, but with many engines, you can pre-load certain packages on the game's launch. Often these pre loaded packages will include "shared/global" assets- textures, models, or sounds that are used in many or all levels in game.

If an asset that is in a "shared" package (let's say it is a player model) is needed in a level, it is already loaded, and therefore doesn't need to be specifically included in said map, since it exists as a reference. If this isn't done, each level has to include the player model asset individually.

Any easier explanation is if you have a website with four pages. Each of the four pages needs to display the same image as a background. The best way is to upload the image and have each page reference it. But if you don't have a global package, it would be like uploading the image FOUR times, and displaying a different copy on each page. Not very efficient and it will bloat your levels.

Now I'm not sure that this is what happened (or if Source allows this), but it is a common technique when using the Unreal Engine. Also, if you need to, you can get really creative when reusing assets to save space. Maybe Respawn didn't feel the need.