Please correct me if I'm wrong, but I assume you mean Nix doesn't have access to the build DAG that may exist inside the hermetic environment of individual Nix builds? If so, that's true, because Nix doesn't do that level of granularity unless you have a way to translate such DAGs into Nix derivations.

But Nix certainly tracks dependencies between Nix packages, and have knowledge about what packages need to be rebuilt if you make a change somewhere. Some of these packages might build config files, while other may build Chromium, ie wildly different CPU+mem needs.

Right, I'm arguing this is the wrong abstraction level, and that only the build system can make correct container sizing decisions.