I'd been suffering from piriformus which also contributed to my tight hamstrings and lower back pain. So the main purpose of buying the Pro Fit shorts (and Pro Running tights) was to prevent injuries as from the images and descriptions, the pro models seem to cover hips and hamstrings (tights). I have to say since I wore them from this summer, I have never suffered piriformus that much as I did before even after speed workouts and 2-3 hours long distance runs - my hip used to be so sore at night after a hard workout that I couldn't sleep well. I ordered small (5'4" and between 110-116 lbs) for both and found small is perfect fine for me. They also make me run faster as I don't have to worry about my injury any more. I try to wear tights more often as they give me more support. When it is really warm or I have to race, I wear shorts, but sleep in my tights for recovery. The next morning I'd feel nothing from my hard workout or race on the previous day. As a matter of fact, I just ordered a pair of the Pro Running shorts (knee high) and Expert tights. They are absolutely great products for me in my situation.
Also I have relatively big legs (marathon training and racing don't help me), but the small shorts (and tights) are fine for me.