Lack of muscle can contribute to the problem, but there can be a number of underlying causes for sidedness: Interfering more in one direction than the other (leg conformation problems greater on one side than the other or inappropriately asymmetrical hoof trim), back problems, injuries, and habit to name some very obvious ones. A lot of racehorses develop a physical asymmetry from being preferentially worked in one direction (not all trainers conscientiously train in both directions, and races generally are anti-clockwise where we live), riding horses can get similar problems from preferential work in one direction more than the other (and from the diagonal not being changed often enough and applied 50:50 during posting trots on a trail etc), from ill-fitting saddles, and from carrying one-sided riders.
Muscle development is helped with careful feeding and conditioning of the horse. Work on soft footing helps tremendously, on the lunge or riding (or driving). Don't do too much too soon, especially on soft ground: Gradually work at increased paces. Undulating terrain is helpful on trails, and figure riding and lateral exercises in the arena - for example.
Soft footing will exacerbate interference problems, so go gently and use protective equipment until your horse's underlying issues are sorted and it stops hitting itself. Interference is most pronounced at extended paces: So a working canter is often (but not always!) less of a problem than a flat-out trot.
Make sure there are no underlying injuries and gear fitting problems before doing any exercise programme!
SueC is time travelling.