Less internal volume means more mids.
Not necessarily, but even then you should be looking at internal volume per speaker.
Not so. The sound waves bouncing around in a closed back cabinet don't know which speaker they came from (#1, #2, # or #4), or even how many speakers are in the cabinet. The volume of the cabinet is what matters, not the volume per speaker.
The larger Mesa cabinets have more volume and hence lower resonant frequency = lower frequency bass thump.