Have you ever heard of a stat called TS%? ( TS% = Pts / (2*FGA+.88*FTA))
Kevin Durant had a TS% of 65% this year, which is basically the highest TS% in history for a guy who shoots as much as he does. LeBron's was 64%, also historically off-the-charts.
Melo's was 56%, which is respectable considering his role on the team, but pretty average among 25ppg+ scorers.
Essentially, every time Durant/LeBron take a shot, they score 1.3 pts. Melo, Wade, Kobe, Pierce, etc average about 1.1 points per shot. That difference alone is worth about 4 points a game, which is huge over the course of a season.
Oh yeah, Durant was also the leading scorer on the Olympic team. Melo had one game where he hit like 10 threes in 15 minutes against a terrible team, but Durant and LeBron were the best players on that team, too.