Help!

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Ok so I just made this loaf with the recipie from lesson II. I tried it with a "Tiger Bread" topping and I think that needs work but my real concern is that my loaf is rather...flat. I am happy with the color, taste and crumb but it seems awfully short and I tried a second batch and it is responding the same.  What can I do different.  It doubled in size both after the initial kneading and after I shaped it and let it rise for a hour before baking.  What have I done wrong?

Over proofing, maybe?

On your next bake, try proofing(the final proof) for a shorter period. Maybe like, just 3/4's of the time of the past two attempts.