The diversity of training is the key to building up the muscles of your horse! You need a good sequence of training days that alternate between training stimulus, exercising without the weight of a rider, or recovery days. Neither the direct repetition of training stimuli, nor too many breaks or easy days lead to the muscle growth of your horse.
A good technique does it! The hardest point: Only physiologically suitable exercise gives the horse muscle growth where we want it. Unfortunately, there is no shortcut for this. To quote the famous German trainer, Paul Stecken: `Correct riding is enough´. That this may be difficult is not a problem; riding is lifelong learning. On the way, you collect a lot of knowledge from which your horse may profit directly.
Engage your horse mentally!! The most ideal physical training program is not worth much if your horse is not mentally involved. Pay attention to his signals, even small ones, which might show unhappiness or stress and prevent your horse from building muscles! There is always a reason.
Keep in mind: It is never only about muscle building, but also about the tendons, ligaments and fasciae of your horse.