Albert Barnes Commentary 1 Samuel 9:21

"And Saul answered and said, Am not I a Benjamite, of the smallest of the tribes of Israel? and my family the least of all the families of the tribe of Benjamin? wherefore then speakest thou to me after this manner?" — 1 Samuel 9:21 (ASV)

The tribe of Benjamin, originally the smallest of all the tribes (Numbers 1:36)—if Ephraim and Manasseh are counted as one tribe—had been nearly annihilated by the civil war recorded in Judges 20.

It had, of course, not recovered from that terrible calamity by the time of Saul and was undoubtedly the smallest tribe at that time. Humanly speaking, nothing could be more improbable than that this weak tribe should provide a ruler for the mighty tribes of Joseph and Judah.