Relays aren't primarily designed for the most efficient exchange of information - but an efficient enough exchange of information such that:
- the try-notry decisions can be reasonably made by 3NT; the slam-noslam decision can be made before 5trump, and so on;
- if at all possible, all information is one-way, *and* the described hand is dummy.
Relay breaks tend to be set up to invert the one-way direction of information flow, and if designed well, also invert who becomes declarer, because having the described hand declarer is more damaging than having the lead go through the strong(er) hand.
Note that that "one-way" thing (and, of course, the "stop on a dime, out of nowhere" thing - but I'm referring to GF relays, as that's what I know) is what the RAs that regulate relay systems are going after, because it is such an advantage.
So, it's an optimisation problem, and "most efficient use of space" is *not* the optimising criterion (though, of course, knowledge of the way of most efficient use of space is important in cramming everything necessary into the first go-nogo decision space - up to 3NT).
Asking bids do similar things, but require more of the optimisation to be done at the table ("I need to know these three things, and if the answer to X is Y this one more - how do I get the information I need short of <bail point>?") with the exchange of not having to go through the motions every time you want anything on the track, and giving that information to the opening leader (and, of course, having different information available, and therefore, using different go-nogo judgement).
When I go to sea, don't fear for me, Fear For The Storm -- Birdie and the Swansong (tSCoSI)