  The decay rates of $D^+\to\pi^+\pi^0$ and $D^0\to K^+\pi^-$ recently measured
by CLEO give the ratios $R_1=2|V_{cs}/V_{cd}|^2\Gamma(D^+\to
\pi^+\pi^0)/\Gamma(D^+\to\bar{K}^0 \pi^+)=3.29\pm 1.16$ and
$R_2=|V_{cs}^*V_{ud}/(V_{cd}^*V_{us})|^2\Gamma(D^0\to
  K^+\pi^-)/\Gamma(D^0\to K^-\pi^+)=2.92\pm 1.34\,$. Both are about three times
of those expected from SU(3) symmetry. We show that, in the large-$N_c$
factorization approach, such large SU(3) violations can be accounted for by the
accumulations of several small SU(3)-breaking effects. An important requirement
is the relative magnitude of the form factors, $f_+^{D\pi}(0)>f_+^{DK}(0)$.
