Phil,
Most 72 bass PA accordions I've seen look as though they could easily accommodate an extra 4 rows of bass buttons without any trouble at all. In fact their often lop sided appearance suggests that the bass side of the body is underused.
As far as 96 bass goes, in my experience the spacings between rows and buttons are usually exactly the same as on a 120 bass.
I don't know if you are referring to any specific make or model, but it is possible to buy accordions all with the same number of piano keys (and therefore identical in body length), but which have anything between 12 and 96 basses in them.
French CBA boxes of identical shape and size often have options for 80 or 100 bass (5 rows of basses), or 96/108/120 basses (6 row). I can assure you that the spacings between the rows and individual buttons are identical. On the odd occasion they'll fit a slightly differently configured bass side on to take a standard 96 bass up to a special order 120 bass model, but that is the exception rather than the rule.
From a personal point of view anything bigger than 96 bass just isn't necessary for the styles of music I play, and in fact after playing 96 bass for nearly 20 years I have trouble reaching all the "corners" of 120 bass instruments. People who play for vocalists or classical music may require 120 bass, but I suppose it's all down to personal requirements.
As a general rule the fewer the bits the lighter the weight, and as Donn has highlighted, a 72 bass requires the same number of reeds as a 120, it's just that the 120 needs more rods, levers and buttons. One other advantage with 96 bass and less is that smaller instruments can be easier to play for people of short stature. The trade off is the reduction in range on the treble side.
FWIW I have a 96 bass accordion which is actually slightly heavier than a 120 bass I have of a different make. Another consideration, which I don't think applies to you, is that if you are of a corpulent disposition a small heavy box can bounce around and misbehave when you're playing hard, whereas a full sized model tends to stay close to the body on account of the greater length. I sometimes end up doing the Lambada with the 96 bass, even when seated, but the 120 basses prefer dancing in close. The chairs I sit on prefer the lighter boxes, as they don't creak so much!