  We have computed in [arXiv:1107.4320 (hep-th)] the glueballs spectrum in a
certain sector of the large-N YM theory by solving by a change of variables the
holomorphic loop equation for cusped twistor Wilson loops supported on certain
Lagrangian submanifolds and by evaluating the correlators of surface operators
supported on these Lagrangian submanifolds. We have shown that the correlators
of composite surface operators of length L reproduce in the large-L limit the
leading logarithms of perturbation theory of the corresponding glueballs
propagators, including the correct anomalous dimensions. In this paper we show
that the correlators of surface operators match in the large-L limit the
stronger constraints arising by the operator product expansion, according to
Migdal technique of computing the spectral sum over the glueballs including the
subleading asymptotics given by the Euler formula. Finally, we discuss
correlators of surface operators for finite L.
