I have a 30-31" waist and the Medium size fitted perfectly.
The greater trochanter is that part of your femur that forms the boney prominence on the side of your hip. It is subcutaneous; that is, it sits directly beneath the skin with little to no fat or muscle overlaying it. When elderly people fall onto their side, it is the greater trochanter that is prone to fracture. Falling heavily onto concrete in the skate park is also a risk. Greater trochanter fractures are very bad news, no matter what your age.
There are 3 side-pads on each side of these shorts. When putting these shorts on, yank them up so that the waist band is above your belly button. In this way, the middle pad will sit directly over your greater trochanter on each side.
The upper rim of your pelvic bone is called the superior iliac crest. It is usually easy to feel; when you put your "hands on your hips", the length of your index finger typically is resting on the crest. With the shorts yanked up, the upper side-pad will sit directly over the crest.
In this way, these 2 prominent bones will be maximally protected, even though you might feel a bit of a dork. But hey, no-one needs to know.