Wim Wakker writes about this much more eloquently than I in his section on "Reeds" and air pressure. Getting back to the original post, the advantage of additional folds is additional time - time to play a phrase or a chord.
Ok, let's agree then that larger reeds consume a greater air flow but the pressure conditions are the same for small
and large reeds while playing. The on-set by larger reeds however may be slower. Concerning folds again...mostly
the difference between 6 or 7 has very little practical significance but of course mainly depending on the music ...if
playing single notes 4 folds ( common among Victorian englishes) may be quite sufficient even with an anglo but you
hardly find less than 5...if playing multi note harmonies with an anglo you may rather need 10 than 7. It is a matter
of cost, production set-up, and tradition.
The results from depth of folds are complex. If construction is exactly the same otherwise deeper folds are expected
to result in less stability ( greater flexibility if that is wanted...) and less volume but it depends on how extractable the
construction is. I guess that when you said: " I gained about 3 inches of travel without adding any additional folds".
you meant that you could extract the bellows 3" more. Or? If so...are you sure that the total volume was any larger
at all? and when you play with the bellows continuosly rather closed, the volume resource likely is smaller with the
extra deep folds, meaning that you will have to do more frequent bellows reversals. Or? Have you measured it? I'm
still confused regarding this : "deeper folds...helps compensate for the extra force required to operate larger volume
instruments". My experience is rather that a wide bellows with shallow folds usually becomes more stable and that
helps operate larger volume instruments...seemingly the opposite...but maybe I misunderstood what you said...