Utility doesn't have to be proportional to the amount of some particular kind of physical stuff in the universe. If the universe contained 1 paperclip, that could be worth 2 utilons, if it contained 2 paperclips then it could be worth 4 utilons, if it contained 20 paperclips then it could be worth 2^20 utilons. The box would then double your utility each day just by adding one physical paperclip.

I still think these kinds of considerations are worth thinking about though. Your utility function might grow faster than a busy beaver function, but then the doubling box is going to have trouble waiting the right length of time to deliver the