Question:

Where is the happy medium here? If horns work best when using a small driver, why not just keep getting smaller? Go down to a 4" driver. Don't stop there, go with 2".

The problem is that the smaller you go, the further you must thrust the cone. Seems to me the compromise is size of box verses depth of bass. This is basically the same compromise as a non horn bass speaker.

That brings me back to thinking size matters.