I'm sorry to tell you that you are terribly misinformed.
Athletes get stronger with yoga, because improved flexibility helps them to do compound exercises with better form, and that allows them to activate the correct muscle groups for better leverage. In other words, they have the same muscle mass - they can just activate those muscles more effectively. This is most likely what happened to you, unless you were so unfit when you began yoga that even simple poses worked the hell out of your muscles.
It's good that you got stronger (for the reasons I explained). But unless you are doing some hardcore yoga, it is extremely unlikely that you actually gained muscle mass. (As proof, you can look at any long-term yoga instructor who wasn't muscular to begin with.)
Athletes get stronger with yoga, because improved flexibility helps them to do compound exercises with better form, and that allows them to activate the correct muscle groups for better leverage. In other words, they have the same muscle mass - they can just activate those muscles more effectively. This is most likely what happened to you, unless you were so unfit when you began yoga that even simple poses worked the hell out of your muscles.