  We propose an expression for the classical limit of diagonal form factors in
which we integrate the corresponding observable over the moduli space of
classical solutions. In infinite volume the integral has to be regularized by
proper subtractions and we present the one, which corresponds to the classical
limit of the connected diagonal form factors. In finite volume the integral is
finite and can be expressed in terms of the classical infinite volume diagonal
form factors and subvolumes of the moduli space. We analyze carefully the
periodicity properties of the finite volume moduli space and found a classical
analogue of the Bethe-Yang equations. By applying the results to the
heavy-heavy-light three point functions we can express their strong coupling
limit in terms of the classical limit of the sine-Gordon diagonal form factors.
