Small in stature, by design
The Arabian is deliberately not a big horse. Breed standards put most individuals between 14.1 and 15.1 hands, and anything much above 15.2 hands is considered large for the breed. In weight, a fit Arabian typically comes in between 800 and 1,000 pounds. Everything about the frame is economical: a fine, dished head on a curved neck, a level croup with the tail carried high, and one fewer rib and lumbar vertebra than most breeds, which is part of why the back is short. Owners used to bigger horses sometimes look at an Arabian and underestimate him — which is exactly the mistake the rest of this answer exists to correct.
Why the Arabian carries above his weight
Few breeds carry weight as willingly, pound for pound, as the Arabian, and the reasons are structural. His bone is exceptionally dense for his size, his back is short — a short span bears load better than a long one — and his hindquarters and loins are strongly muscled. On top of the structure sits the temperament and conditioning of the original desert horse: this is the breed that dominates endurance riding, covering very long distances under saddle while staying sound and recovering fast. A fit Arabian in regular work is a concentrated athlete, and riders regularly find that a 15-hand Arabian feels more substantial under them than a rangier horse two inches taller.
The numbers, honestly applied
The standard guideline still applies as a starting point: a total load — rider, saddle and gear — of around 20 percent of body weight. For an Arabian of 800 to 1,000 pounds that works out at roughly 160 to 200 pounds all-in. Within that, judge the individual. A fit, mature Arabian with a strong back, in regular work, sits comfortably at the top of his range and may manage a somewhat heavier balanced rider for moderate work better than an unfit horse manages less. A young, green or out-of-condition horse should be matched well inside the guideline. The percentage is the beginning of the conversation, not the whole of it — fitness, back strength, saddle fit and rider skill all move the real answer.
Matching a rider to an Arabian in practice
Have his weight from a tape or bridge rather than an estimate, because guessing an Arabian's weight from his height under-reads him in both directions. Check the saddle fits his short back without pressing beyond the last rib, and build a heavier rider's work gradually, watching his back after rides and his attitude to being tacked up. Endurance-bred lines in hard condition are the strongest carriers in the breed; show-ring lines kept soft are not, even at the same height. Ask a seller what the horse currently carries and over what distances — with Arabians, the work history tells you more than the tape measure.
Keeping his records up to his reputation
An Arabian's value as a weight-carrier lives in his conditioning and soundness history, so those records are worth keeping properly: weights, fitness work, vet visits and any back checks, all in one place. KinsLe's Pet Passport holds an Arabian's records together in one portable packet, ready for vet visits, travel and boarding — the paperwork finally organised enough to match the horse.