I do know that Conjuring the Mystic Tower as written is not as large as it could be if a new spell was reinvented, trying to maximize the space, but I've always seen that as the spell inventor wanting a specific size, rather than designing a spell at a given magnitude to make the largest tower he could. I'm re-reading your message here though, and I don't fully understand what you mean. Are you saying the empty space within shouldn't count for the volume created, or?