I salute your desire to run a tight ship. If there were a leak in the nix store, I'd never know until I my garbage collection stopped solving the problem and I'm sure it would be at the worst possible time. If such a leak exists and is found, it'll be by someone running a tight ship.
Instead of running a tight ship I spend a lot of time dreaming about alternate computational universes, and one I particularly like is where new hard disks come pre-loaded with a fragment of bits deemed culturally valuable. Wikipedia and a well curated subset of nixpkgs would be a fine start to such an archive. In this world your files don't grow/shrink to consume/yield empty space, but rather the boundary between your data and that drive's shard of the public archive shifts in one direction or another. This way you or somebody in your neighborhood is likely to already have the file you need, so you can get it from them instead of the internet. Better for being able to roll with the punches if the internet is partitioned.
I don't worry about the size of the nix store because according to this weird fantasy of mine it's on the side of my disk that shrinks when I add files to it; it's not the contained object, but the gas that expands to fill the rest of the container. Not by accident, but as part of a redundant worldwide distributed cache that we put together after deciding that servers controlled by people we don't know are not to be relied on.
