Jamieson, Fausset and Brown on Numbers 11:21-23

21-23. Moses said, The people, among whom I am, are six hundred thousand ... Shall the flocks and herds be slain for them, to suffice them?--The great leader, struck with a promise so astonishing as that of suddenly furnishing, in the midst of the desert, more than two millions of people with flesh for a whole month, betrayed an incredulous spirit, surprising in one who had witnessed so many stupendous miracles. But it is probable that it was only a feeling of the moment--at all events, the incredulous doubt was uttered only to himself--and not, as afterwards, publicly and to the scandal of the people. (See on Nu 20:10). It was, therefore, sharply reproved, but not punished.