Mini-box picture and aspect/size

With the coax attached straight to the TV, and without the mini-box, HD channels fill my TV screen perfectly, without "spillage" outside the edges of the screen.  With the mini-box (using all combinations of image variables on both the TV settings and the mini-box settings) I cannot get an image to fill the screen without losing the edges, top, bottom, and/or sides.  Is this normal?