Verses one and two should be read together, principally because it demonstrates the continued conceptual divisions among Mosiah's people. He calls all the people, and then we have the discussion of Nephite and Mulekite peoples.
Even though Benjamin had ushered in a uniting of the peoples, there were still divisions, which would almost certainly have been along family lines. As we have seen at various places in the Book of Mormon, kin organizations underlay the larger political structures. Verse two is yet another indication of the persistence of kin organizations and sensibilities within the larger political units.
The demographic information is also important to remember. Even though we are used to understanding the Nephites and Lamanites as the two major peoples in the New World, we must remember that the definitions of those terms are collective for the multiple kin organizations attached to the larger political units (and indeed, that Lamanite frequently means little more than "not-Nephite"). Within the political Nephites, the lineal Mulekites were the more numerous.
The comparison used here is that there were "not so many" of the Nephites as the Mulekites. It is useful to recall the reasons for this population division, according to the way this commentary is reading the text.
First, we should remember that the Lehites and Mulekites leave Jerusalem at roughly the same time. While this is a difference in time, that difference in and of itself is not sufficient to warrant a large population difference. While we do not know how many Mulekites left together, we may presume that it was likewise a single ship-full of people, and once again, there is little in the beginnings of the peoples that would account for the differences.
The possible reasons for the population distinctions between the descendents of Nephi and the descendents of Mulek are:
Historical: As noted in the chronological discussion following Mosiah 7:1, this reuniting of peoples likely took place in 106-5 BC.