In another thread there is discussion about Sonny putting 100 button men on the street to kill Sollozzo. That's a lot of button men on relatively short notice, and it shows the Corleone family had significant muscle at the time. I am wondering, with Vito getting shot and never really fully recovering, whether Sonny's bad leadership caused defections
from the family. Also once Sonny was killed this must have caused further weakening of the family due to uncertainty on the street. This is implied by Moe Green who tells Michael
the Corleone family "don't have that kind of muscle ANY MORE."
And also confirmed by Vito -- twice -- once in a deleted scene where Michael upon his return from Sicily tells Vito his compromise on drugs was a sign of weakness, and Vito says the family really is weak, and later when Vito says outright that
Sonny was a "bad Don."