Lotttsss of trotting! Try teaching what they call the “long and low” it will really build top line and hip muscle!
What Bella said plus do hill work, grids and lunging. For feed it will take time but good quality hay, increase the protein and fat to help add weight and muscle
I would recommend doing transitions, walk, trot, canter!! Trot poles and raised trot poles are brilliant as well for gaining muscle