We talk about a quarter inch seam, but remember, you don't measure the seam, you measure the resultant patch. take a minute to double check your seam setting:
http://www.quiltingboard.com/tutoria...ce-t89997.html
If your starting patches are the correct size, and you think your seam allowance is correct, you may need to take a slightly narrower seam because there are additional seams in the patches taking up room.
Another thing to check is that you are not ironing in a tiny crease in the seam when you press.
The only other thing I can think of is if you are now using steam and the patches are shrinking a bit. This can happen if you don't prewash.