I have done binding this way and have had the same issue. There were many good tips from the Quilt Board on the post I did on binding below that may help you. When you square the quilt, leave extra batting to make the binding fuller so when you turn the binding to the back it just covers the stitch line from attaching the front binding. I also use wonder clips to hold the binding. They have the marks on the clips that help you see that the binding stays the same width around the quilt as you attach it. (read post below to clarify)
My Machine Binding done this way is never full. Any tips ?