A horse that rides big is one that feels larger than is when you are on top of it. "She's only fourteen hands, but she rides much bigger."
Riding big is a result of either of a combination of two factors: The width of the horse's body - i.e., a stockier build or an extended or elevated action (a big gait or big stride).
Needless to say, a horse can also "ride small," but you don't hear that said
as often. Riding big is often used as a selling point for shorter horses.