When you sew, you are looking to get the same amount of top thread on the top and bobbin thread on the bobbin. The zig zag stich has a lot of top thread so you are going to have a lot of bobbin thread showing. this is normal. What you want is a smooth stitch on the front and no bobbin thread showing on the front. You also want the stitches to lay flat and smooth. You don't want the tension so tight that it pulls the fabric together. Most embroidery units, which often use a zigzag type stitch tell you to aim for about 30% top thread to show on the back. I personally think that all your samples have the top tension too high. There's NOT ENOUGH bobbin thread showing and the fabric looks like it's slightly pulled. In the end, however, adjust your tension so the front looks good!