OK, here's the math. The diagonal measurement of a square is roughly 1.41 times the length. So the diagonal measurement of a 10" square is a bit over 14 inches. When you cut the X in the square you cut the diagonal in half, so each HST is 7" less the seam allowance.
But I'm with Jan. Even if I starch everything, I pull these bias pieces out of shape. Most people have no problems with them, however.