Great post, arten. Physics will tell us that the shorter the span between two supports, the more weight that span will bear. When distance is a factor, the span needs to be arched upwards (think of bridges, and ceilings of cathedrals), distributing the weight so that it “pushes” out and downward onto the support. Usually the greater the distance of the span, the heavier the width of the support.
So… the denser leg bone of the Arab and the genetically shorter span between the base of support (front and hind legs) argues that this is the reason Arab horses so easily can carry higher weight vs. their size as (read this as a caveat, please) a general rule. Denser leg bones are the hallmark of the draft horses, but lightness and endurance for distances are (for the most part) not – which is why you will rarely see a draft doing endurance except for the draft ponies (who are few in number, but still out there doing endurance (50 miles +) successfully. 
And yes, I’ve seen far, far, FAR too many horses come back from an endurance ride with sore backs. 
That’s the reason the vets will probe the saddle area and loins at the vet checks – “tack off” is often called for at certain checks, principally for the vets to check the horse’s back and sides for tack issues. The horse has to exhibit obvious back pain since the vets only lightly test.
And I agree with arten – a lot of common market saddles aren’t designed for the Arab’s shorter back, something that endurance saddle mfr have understood and designed into their products. If you want your saddle to fit your Arab, you might be best to look at those designed for an Arab. Be careful of those with the flat supports pushing back towards the loins – while theory says to broaden the base of support to disperse weight on a greater portion of the horse’s back, these supports often come too close to the loins on a short-backed horse for the animal’s comfort.
However, back issues are not always the tack – sometimes the harshness of the terrain (sand, rock, hard climbs) will cause aches and soreness. Saddle issues will usually manifest themselves as soreness over the small of the back from withers to loins; terrain/conditioning/fatigue issues show up in the loins, sacrum, and a change in gait movement.
All rather interesting.
I can’t comment on the length of stride as a point in this – I’ve seen so many different stridings on horses that have succeeded or failed that … I can’t make a determination one way or another if it matters to the ability to carry weight. Also, from my understanding, the spring of the rib cage is generally looked upon as room for the lungs to expand, not for weight distribution or the length of stride. I’ve see a host of slab-sided Arabs do just as well as the well-sprung Arabs. Slab-sided horses/ponies (like donkeys) get a hard knock because, lacking a wide barrel, the rider’s legs tend to hang down straight from the rider’s pelvis which some riders find very uncomfortable and annoying. That’s all. Just personal preference.
Check out these pictures of a 
