  The recent precision measurement of the relative yield of $D^+D^-$ and $D^0
\bar D^0$ meson pairs at the $\psi(3770)$ peak in $e^+e^-$ annihilation is
about 3.7 sigma away from a straightforward theoretical estimate. It is argued
that combined with an even more significant deviation of similar ratio for the
$B$ mesons at the $\Upsilon(4S)$ peak, this provides an insight into the
structure of heavy mesons and requires both a form factor, corresponding to
their finite `size', and a strong interaction between the mesons mesons in the
isovector state $I^G(J^P) = 1^+(1^-)$. It is also argued that the data for the
$D$ and $B$ mesons are possibly compatible with the heavy quark limit of the
same meson-antimeson strong interaction in both systems.
