Is 6gb a lot of prototyping stuff, or an average amount?
For 3D? It's lower-average. It's almost entirely comprised of textures that are frustratingly uncompressed or not-frustratingly oversized.
Models themselves are always teeny, but most prototyping assets have a bunch of different textures to them to give full PBR quality, and they tend to put them in tga format for some reason, even sometimes PSD, when either PNG will do and be far more compressed (if there is no alpha channel), or ZIP-compressed TIFF will be much more compressed if there is an alpha channel. Sometimes they even use 16 or 24 bits per pixel instead of 8, which is incredibly excessive for anything that isn't print media, in my opinion.
A lot of times you have things like normal maps that are 4k pixels squared and they'd be just as useful in an HD context if they were 1k pixels squared in my opinion. So when I'm optimizing stuff like that, I tend to take the original tga (that would be a 64MB file) and move that to something closer to a 2MB PNG, depending on how complex the normal map is. Other things get less reduced, but overall the compression I wind up with is about 20%.
I'm also not a fan of height maps most of the time, or occlusion maps. When using Ambient Occlusion post processing, I really don't see the need for baked-in occlusion textures most of the time. Metalness and specular maps can also be either shrunk or removed, depending on what is being represented. Worst case those can be reduced from 4k pixels squared to 1k instead, because those tend to be something where you don't need that degree of precision even if you're looking at it up close.
In the case of the stuff here, there's a lot of stuff I'm not bothering compressing because I'm just going to toss it out anyway. Ultimately I'll be surprised if the final game cracks 1GB, but you never know. Prior to going 3D, music was the largest source of file size. One way or another, music/sound/textures are 99% of the file size of any sizeable game (movies being another piece in a AAA project if those are prerendered rather than in-engine). Shaders, special effects logic (not their textures), game code, and the like are all tiiiiiny by comparison. Under 5MB, symbol files for debugging not counted. The actual engine code itself adds another 20-30MB on top of that, but that's it.
So yeah, textures are the big thing.