Thank you Mikolaj, very informative. The way you describe your mounting technique is very similar to the method I mentioned as a good technique. I did not choose that lady because she is bigger, that has nothing to do with it, she is in fact an experienced rider, her size is no excuse for improper mounting techniques.
I brought this up for the exact point you are making, mounting solely with your lower body is not correct either. It places all the weight into one side of the saddle, exactly the same as pulling yourself up using the horn.
The method (for both english and western) I am trying to promote involves all four human limbs. The left arm, balances the rider and is used to push without using the tack, the right arm, is used to pull (or push depending on the height) simultaneously with the left leg, this minimizes the pull on the saddle and the horses back, and the right leg, is used to push off and aid with momentum. It is a very balanced method for both the horse and rider.
Mounting blocks are an amazing tool, but they are not always present and even if they are, knowing how to mount correctly is still extremely important.