Yoga is made up of a variety of techniques and practices aimed at integrating mind body and spirit. The goal is to reach a state of oneness with the universe or enlightenment. There are many paths of yoga. Hatha Yoga, which is often thought of as “yoga” in the Western world is just one of them. Yoga is a practice of personal exploration which can be incorporated into any religious belief system. http://www.yogabasics.com/learn/yoga-101-an-introduction/
In the West yoga has become mainstream and is known by many as a purely physical form of exercise for health, disconnected from the spiritual aspects. There is so much more to yoga than this.
If your yoga practice is focused solely on physical exercise, I encourage you to dig a little deeper. Slow things down a bit and practice the postures in a slow, purposeful way. Focus on your breath and be in the moment. Feel the joy of total relaxation during the closing meditation..
Finally, there should be a deep sense of mutual respect between teacher and students. If that is not present or if a teacher tries to shame or belittle you for your size or current state of physical fitness, it’s time to find a new teacher.
espirational.com 
I’ve never tried yoga. As you’ve written my friends who do it treat it as a pure form of exercises ‘good for your spine’.
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