Capacity is controlled by the strength of the horse, Al-Buraq, my horse is a Tier 5 Horse with high strength, and also on the saddlebags. I bought two saddles with different bags and checked the carrying capacity of my horse and it has a massive difference. Horseshoes make your horse faster, a saddle makes it easier for Henry to stay on the horse, the bridle helps steer the horse, particularly around tight corners, and the armor-cloth thing I can’t remember the name of protects the horse and raises it’s courage. The more saddlebags on the saddle increase the carrying capacity but lower the speed. Just like armor and clothes, the horses gear comes in different tiers that improve your horses stats significantly. When I was still using Pebbles with Noble Horseshoes, he was faster than every other horse until I got a better horse with better stats but used the same equipment.
To clarify, With full gear, Pebbles was faster than Al-Buraq, a tier five horse, without gear, but with gear, Al-Buraq was nearly twice as fast and carried almost three times the weight.