I heard that when Carlo Gambino was running the Gambino family, the group consisted of around 500 made men, and it was the largest family in the United States. And the Philadelphia family had around 80 members at its peak (sounds a little small, but considering the fact that its the only Mafia family in Philly, easily makes up for it). Today the Genovese and Gambinos have considerably fewer members, around 200 a piece or so. One of the reasons is due to cultural assimilation.

The height of Italian immigration to America was around the 1870's to 1920's. Many of them were poor, unskilled, and had limited opportunities offered to them. Often times the wealthier Italians were the gangsters, so many young men gravitated to what they considered an way of upward mobility. That is not to say that all Italians in this era resorted to crime, but there were enough of them to keep the organization pumped with new blood. Over two to three generations, Italian Americans are more likely to go to college, land well paying jobs, move out of the old neighborhoods, etc rather than resort to joining the criminal underworld. Plus its easier than ever for law enforcement to infiltrate these families with various means (informants, undercover, etc). So mobsters are more reluctant to allow outsiders into their group. As a result, Mafia organizations have been withering over the last three decades or so.