Good question... Er, I've just taken a look, and it doesn't keep a hash, it's an array. oops. So in effect it is just a list of dependent files and my specification of the problem was wrong. sorry ;)

Are you calculating the full closure for dependency files, or just keeping a 1-deep list?

It is a full closure -- but that's not what's taking the memory here. I plan to refactor that alright -- but not just yet...