Thread thickness, cutting, and seam width ALL account for variances in the seam widths. Many good tips have been provided!
Here is my test BEFORE EVERY PROJECT:
1. Cut 1-1/2" strips of fabric (about 4-5" long). Make sure they are uniform & straight.
2. Sew with your "normal" seam, sewing 3 pieces side-by side.
3. Press the seams open, or to the side - which ever method you will be using in your project.
4. DO NOT measure the seam width itself - measure the WIDTH of the unit across the 3 pieces - this is actually what matters, right?
5. If the sample is too small, your seam is likely 1-2 thread widths too WIDE.
5b. If the sample is too big, your seam is likely 1-2 thread widths too NARROW.
6. Try the test again with fresh fabric, either moving the needle slightly right or left, or lining up in a different place according to your presser foot.
7. The ultra-fine threads such as Bottom Line and Masterpiece, Aurifil and others DO provide a finer seam - however, evaluate the use of the project, and the amount of quilting you plan for the total strength of the thread in the project. For instance, if I am making a wall-hanging or art piece, it won't take a lot of laundering, wear-and-tear. Fine threads will work well. If I plan to do a lot of micro-quilting, the stitching through all layers will support the seams. However, if I plan moderate quilting (resulting in a softer, more pliable quilt), I need a stronger thread in the seams so it will hold up to everyday use and laundering.
FINAL comment - some sewing machines do not permit you to move the needle in fine increments. The feed-dogs may not like to feed the fabrics well when you are sewing an extremely narrow scant 1/4" seam. TRY THIS TECHNIQUE: When cutting, do not line up the raw edge between the cross-hairs of the ruler as you cut, but INCLUDE the PRINTED LINE as part of the cut. (Set the ruler on top of the fabric, allowing the printed line to set OVER the edge of the fabric.) The printed line will provide approximately the width of the thread in the seam, and you may have more consistent success!
Jan Krentz, Author, Designer, Instructor - www.jankrentz.com