Yes, OMs are usually shorter than bouzoukis (though my Buchanan zouk is exactly the same length as my Oddy 'cello). Zouks and 'cellos are usually longer (about guitar length), OMs somewhere shorter (if they're to be used for tunes, anyway), and Mandolas shorter still - though Tom Buchanan ships his long instruments (625mm) in Zouk, MC and OM tuning, and his short instruments (500mm) in both OM and Mandola tuning. (You can have a bass the size of a uke if you have thick enough strings - in theory)!

I tune my long Buchanan as a true MC, but it spends the bulk of its time capod on the 7th fret as an OM. That way I get nice close frets for tunes in fiddle/mandolin fingering, and nice low notes for accompaniment on the same instrument.

But I do fancy the octave idea. I think. (or do I?)