Jamieson, Fausset and Brown on Numbers 4:34-49

34-49. Moses and Aaron and the chief of the congregation numbered the sons of the Kohathites, &c.--This enumeration was made on a different principle from that which is recorded in the preceding chapter [Nu 3:15]. That was confined to the males from a month old and upward, while this was extended to all capable of service in the three classes of the Levitical tribe. In considering their relative numbers, the wisdom of Divine Providence appears in arranging that, whereas in the Kohathites and Gershonites, whose burdens were few and easier, there were but about a third part of them which were fit for service; the Merarites, whose burdens were more and heavier, had above one half of them fit for this work [Poole]. The small population of this tribe, so inferior to that of the other tribes, is attempted to be explained (see on Nu 3:39).