I'd guess at least part of it has to do with increased core counts. Each instance of the compiler might use a few hundred MB, which by itself is probably fine,

I'd guess at least part of it has to do with increased core counts. Each instance of the compiler might use a few hundred MB, which by itself is probably fine, but multiply that times 8, 16, or 32 and it adds up. And if you're building even a medium-sized c++ codebase you're really going to want to use all available cores.