I currently have a QH gelding that is N/H for HYPP and he is a hard keeper. He eats more than twice what any other horse we have does. However, the horse I owned before him was also N/H and was definitely an easy keeper! Both geldings received roughly the same amount of exercise/riding and are 1400-ish lbs. So while diet restrictions may add to HYPP horses being harder keepers, I don’t think it’s necessarily a rule.