It’s textures.
If you have 15 different wood types in your level, unless they are obviously using the same texture, you’ve used at least 15*3=45 textures. ( albedo, normal and roughness etc ).
On disk, at least, a 2k texture is about 8MB, a 4K texture is about 80MB 
( very roughly, it depends on what the texture is )
If those textures are 4K, which they probably are ( because everyone supplies this without thinking ), then that’s much worse than 45 2K textures.
Textures use up your GPU memory.
If you have a lot of different assets from various sources, I know it can be tempting to drop lots of different things in the map, to make it look great. But you can just as easily get great looking levels using only a very small number of assets. You just have to know how to repurpose them in the right way.
This, a pic from a level I’m working on
uses ONE rock and ONE tree.
I know it’s not arch viz, the point is economy of textures.
If you want to just make beautiful renders of apartments, no problem! Just get a GPU with more memory, and off you go.
But if you want to make an end user experience, ( a program people can run on normal PCs ) you have to economize.
Is it starting to make sense yet?