I'm reading Jerry Capeci's recent bio (Al D`Arco), and he discussed the headcounts of the five families. Capeci wrote that Lucky's original commission decided on the maximum headcounts for each family to keep everyone in line.

Genovese....300
Gambino.....300
Colombo.....150
Bonanno.....125-140
Lucchese....125-140

This doesn't necessarily reflect the current numbers, but is where each family is capped at when recruiting new members. Even though families can replenish the ranks, some opt not to in order to keep a bigger share of the loot, or to avoid diluting the standards. Since the figures posted by other members are lower, I'm assuming the decision to keep the headcount lower is voluntary.