1. Horses and ponies are a single species (a subspecies actually). Heights (measured to the withers) range from 38 to 72 inches. This does not include the heights of miniature horses, which were developed in modern times. I'd expect a similar range of sizes in the centaur population. The average human sitting height of 35 inches, when combined with the average height of Equus ferus caballus of 55 inches, yields an average centaur height of 90 inches (7' 6"). The full range of centaur heights, however, when extrapolated from the range of horse and pony heights, is 62 inches (5' 2") to 118 inches (9' 10").
2. The monster entries in the Monster Manual do not necessarily represent the average specimen of a given creature. They represent the typical combatant of the creature type that one might expect to encounter on the battlefield. In the case of centaurs, the MM centaur represents the "regular soldier" of a fielded centaur force. Like herd animals, centaur society is protected by the larger "bull" males, those in the heavy or draft horse range of sizes. A centaur standing at 9' 10" sounds like it would fit solidly into the Large size category.
3. A PC race entry does represent the average specimen of a given creature. A centaur standing at 7' 6", with a horse body the size of the smallest light riding horse or the largest pony and the upper body of the average human, could well be considered a Medium creature.