  We analyze conditions of applicability of grand-canonical mean-field
Bardeen-Cooper-Schrieffer theory to the evaluation of an interaction energy in
the ground state of small-sized superconductors. We argue that this theory
fails to describe correctly an interaction energy, when an average distance
between energy levels near the Fermi energy due to the size quantization
becomes of the order of the single-pair binding energy. In conventional
superconductors, this quantity is much smaller than the superconducting gap.
