No matter what you weigh, it's important to make sure the saddle fite.
The best way is to put the saddle on the horse, no pad, and look at it. Make sure the front concho is 1-2" behind the back edge of the shoulder blade (this is the proper place to put a saddle on a horse). Does it set level on his back? Does it pop up in back or look "uneven"? Where do the back skirts end? They should be no farther back then the mid-line of the flanks.
Next, look at the front. Does the front of the saddle follow the same angle as the horse's shoulder? Is there at least 1-2" of clearance above the horse's withers?
Lastly, run your hand under the saddle. Do you feel even pressure front to back? Do you feel any tight spots or loose spots? Sometimes it helps to have a friend lay their hand on the seat of the saddle and press down lightly, if the saddle is really light weight.