Why are my biceps getting softer?

My biceps are my favourite area to train. Just 10 years ago I had powerful 17.5 inch guns. This was so satisfying with my 6'4 frame. They stayed consistent in size between 30-52 years of age. They were cutting ribbons and I received loads of compliments on them especially during the summer. They screamed muscular and athletic whenever I flexed them. Now whenever I flex they feel softer and despite doing the same arm workouts they have shrunk by about 1.5 inches. Why is this?
Why are my biceps getting softer?
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