From what I understand, the Luciano/Genovese and Mangano/Gambino families have always been the two large NY families. I've read some things that said the Genovese were initially bigger, while others have said the Gambinos were initially bigger. Certainly during part of Carlo's reign as boss, the Gambinos were the biggest, but not by a significant margin. Not in the way those two families are significantly larger than the three smaller NY families.