The gathering continues

I tried again last night with the little practice piece. First, I put a piece of masking tape on the sewing machine to mark 1/4-inch seam width. Then, I sewed together the two 3 1/2-inch squares. Marking the width helped; the seam was nice and straight. However, the width of the piece along the seam changed to 3 1/4 inches. The machine's tension setting is set to "auto". I'll try playing with that. If it can't be resolved, I may have to have the machine examined by a professional repair person.