The Luciano and Mangano (and the Gagliano, to a certain extent) gangs were bigger than the Bonannos and the Profacis from the start as their gangs were older and able to draw on a larger "talent pool", so to speak. The majority of the latter two were Sicilian natives brought over to America in adulthood, whose families were smaller, but generally more tight knit and familial, at least until the next generation. I would assume that when it came time for recruitment, aspiring members would generally want to join the larger and wealthier crime families of Luciano and Mangano, as opposed to the smaller, and largely foreign speaking, Bonannos and Profacis.