Exactly! Pressing with muscle effort causes pain or discomfort. Body weight feels much better and is easier on your own body as well. Thai Massage lends itself to using body weight since it is generally done on the floor. That makes it easy to shift your body in the perfect position to take advantage of your body weight. This is much harder to do when you work on a massage table since you cannot get on top of the client.
One of the main principles of Thai Massage is to leverage your power as much as possible through using your body weight without using muscle power more than absolutely necessary.