If you are heavy it is suggested not to ride in a treeless saddle
Because a treed saddle has bars weight is distributed over your horses back more evenly. A properly fitted saddle should not offer more than 4lbs of pressure per square inch of horse. If it does, you will see white hairs on your horses back where there was too much pressure.
If you are riding bareback or in a treeless saddle, all of you weight is right where you are sitting and no where else, and if you brace in the stirrups you can add even more weight. As already mentioned is is dire to have a properly made treeless saddle pad made for your brand of saddle. Mixing and matching, like buying a no name pad and pairing it with a Bob Marshall wont work, or mixing a Bob Marshall pad with a Barefoot probably wouldn't work either.
If you weight is more than 175lbs they say you shouldn't ride in a treeless for more than one hour at a time, If you plan on trail riding in one you are supposed to get off and walk your horse for ten minutes between each hour and check for soreness.