you might be sewing too close to the diagonal line.
finished size + 7/8ths inch square
draw diagonal line on back of lightest of the two fabrics
place the two fabrics right sides together, lined up very carefully
sew 1/4" or "scant 1/4" away from each side of the diagonal line.
a scant quarter inch is really about 3/16ths of an inch. line your needle up to sew a true quarter inch, then adjust the needle position one or two clicks to the right. by using a scant instead of a true, the patch is more likely to come out the right size once the triangles are folded open.
dizzy enough yet? :lol:
as a consolation prize, consider this: too big is waaaaaaaaay better than too small because you can trim it back to the proper size. :P