David is portrayed in first and second Samuel as the anti-Saul. As Samuel seeks out the new king in 1st Samuel 16, he first believes David’s brother Eliab to be the candidate based on outward appearance. Samuel is then rebuked by the LORD for this assumption and told that the LORD looks at the heart rather than the outward appearance.
While not explicitly stated, this would indicate that David was not an impressive physical specimen though the text does describe him as handsome. Additionally, as the youngest son he had no inheritance, and as a shepherd he mostly lived outside with sheep. David was not the outwardly exceptional man Saul was.
Neither was David’s heart like Saul’s. Not only David’s superior heart the basis for the LORD’s selecting him, David exhibits behavior that displays the inward differences between Saul and himself.
In 1st Samuel 17, David is indignant at Goliath’s taunting. Rather than shying back because of Goliath’s greater size and experience, David faces Goliath in single combat trusting in the LORD to deliver victory. In this same chapter David states that he has also fought “both lions and bears” in his role as a shepherd.
Although Saul repeatedly tries to kill David after David becomes too popular for Saul, David twice spares Saul’s life when presented with the opportunity to kill Saul.
Later in 2nd Samuel 11-12, after David takes Bathsheba, has Uriah killed, and is confronted by Nathan, David not only acknowledges his sin, but turns his heart toward the LORD and is pardoned by the LORD. Unlike Saul, David is allowed to remain king.
While David was outwardly unimpressive, inwardly according to 1st Samuel 13:14, he was a man after God’s own heart.
For a differing opinion on 1st Samuel 13:14: see here.