Horses become overweight for the same reason we do-they eat more calories than they burn. It isn't any healthier for them than it is for us. Putting your horse on a diet could not only make your horse more active, but save his life. Overweight horses may lead to conditions of lameness, heart and lung problems, lethargy, and a slew of metabolic problems.
Lameness: Lameness is one of the most common health problems in horses. It causes pain for the horse and robs you of time in the saddle. The stress placed on your horse's joints, tendons, ligaments and hooves is directly proportional to the horse's weight. One of the most basic equations in physics (don't panic, it really is basic!) is F = m x a, where "F" is force, "m" is mass (weight), and "a" is acceleration. The force your horse's support systems experience with every step, at every speed, is influenced by his weight. An overweight horse traveling at the same speed as a leaner companion is subjecting his joints, tendons, ligaments and feet to greater stress. Sooner or later, it takes its toll. More strain is also put on the horse's back.
Heart and Lungs: Horses aren't prone to the same problems with fat-clogged arteries and heart attacks as people. But it's still more work on their heart and lungs to move around an overweight body. If you take two relatively unfit horses, one lean and the other overweight, it's not difficult to predict which one will be huffing and puffing sooner, even on a relatively easy trail ride.
Other Exercise Issues: A heavy layer of fat makes it difficult for your horse to regulate his body temperature during exercise. Working muscles generate a considerable amount of heat that the horse needs to lose through sweating, etc., to be able to continue exercising. In warm/hot weather, most people's peak riding seasons, the overweight horse is at greater risk of dangerous overheating and heat stroke.
Metabolic: Fat tissue isn't just an inert, jelly-like mass. Fat cells actively secrete a variety of substances with hormonal effects. Overweight horses can be more likely to be insulin resistant, a pre-diabetic state that results in even more weight gain, abnormally increased appetite, and increased risk of laminitis.
Lethargy: It's always nice to work with a cooperative horse, but we don't want that to be because the horse doesn't feel well. The more overweight a horse becomes, the less active he is and the less interest he finds in his surroundings (except food). Horses are extremely athletic creatures by nature, not normally dull, apathetic and listless. Becoming overweight can rob your horse of that interest in life and movement.
One thing that holds people back from trying to get their horses to a better weight is fear that the horse will have to be "starved" to get there. This simply isn't the case. Your horse can have plenty to eat and still lose weight. The trick isn't how much you feed him-it's what you feed.