It seems to be quite an ancient association, the one between genital size and courage. Here's one of my favorite passages from the I Kings 12. Context: Solomon died, and his son Rehoboam has taken over. The Northern Tribes in particular want the new king to lighten the load of corvée labor (think tax paid in labor) that Solomon used to build his great works.

12 Rehoboam went to Shechem, for all Israel had come to Shechem to make him king. 2 When Jeroboam son of Nebat heard of it (for he was still in Egypt, where he had fled from King Solomon), then Jeroboam returned from[a] Egypt. 3 And they sent and called him; and Jeroboam and all the assembly of Israel came and said to Rehoboam, 4 “Your father made our yoke heavy. Now therefore lighten the hard service of your father and his heavy yoke that he placed on us, and we will serve you.” 5 He said to them, “Go away for three days, then come again to me.” So the people went away.

6 Then King Rehoboam took counsel with the older men who had attended his father Solomon while he was still alive, saying, “How do you advise me to answer this people?” 7 They answered him, “If you will be a servant to this people today and serve them, and speak good words to them when you answer them, then they will be your servants forever.” 8 But he disregarded the advice that the older men gave him, and consulted with the young men who had grown up with him and now attended him. 9 He said to them, “What do you advise that we answer this people who have said to me, ‘Lighten the yoke that your father put on us’?” 10 The young men who had grown up with him said to him, “Thus you should say to this people who spoke to you, ‘Your father made our yoke heavy, but you must lighten it for us’; thus you should say to them, ‘My little finger is thicker than my father’s loins. 11 Now, whereas my father laid on you a heavy yoke, I will add to your yoke. My father disciplined you with whips, but I will discipline you with scorpions.’”

12 So Jeroboam and all the people came to Rehoboam the third day, as the king had said, “Come to me again the third day.” 13 The king answered the people harshly. He disregarded the advice that the older men had given him 14 and spoke to them according to the advice of the young men, “My father made your yoke heavy, but I will add to your yoke; my father disciplined you with whips, but I will discipline you with scorpions.” 15 So the king did not listen to the people, because it was a turn of affairs brought about by the Lord that he might fulfill his word, which the Lord had spoken by Ahijah the Shilonite to Jeroboam son of Nebat.

Two fun things: first, spoiler, Jeroboam splits, the United Monarchy period of Israelite history ends, and Jeroboam and Rehoboam go on to lead the Northern Kingdom (Israel) and Southern Kingdom (Judah), respectively.

Second, notice that they only discuss the thickness of loins among the "young men", and Rehoboam doesn't actually say that part to Jeroboam's face. He just repeats the other, less obscene part.