In 2009-10, LeBron James and the Cleveland Cavaliers won 61 out of 82 games, and in 2008-09 they won 66 games. When James left the Cavs to join NBA All-Stars Dwyane Wade and Chris Bosh at the Miami Heat, some thought the Heat may win over 70 games. While the Heat have won a lot of games, their winning percentage has not really been any higher than the Cavs in the couple of seasons before LeBron's departure, as they won 58 games in 2010-11, 46 out of 66 games in the lockout-shortened 2011-12 season, and 66 games in 2012-13. The Heat have won two NBA championships, but they have not steamrolled opponents in the way one might have expected given they have two All-Stars and the best player in the game. Why?
To answer this, let's look at the 'wins produced' by each player on the Cavs and the Heat during these seasons. 'Wins produced' is a formula popularised and in part devised by economist Dave Berri to measure the worth of each NBA player. If you don't think this formula means anything, you may as well stop reading here. But assuming it is useful, let's see what it says.
Here are the wins produced by the Cavs in 2008-09 and 2009-10 (courtsey of NBA Geek):
LeBron has been pretty consistent, although he produced a bit more during the Cavs years when he was at an age that players (according to the wins produced metric) are more likely to be at their peak. Dwyane Wade is clearly a better 'second banana' than anyone on the Cavs was. But the difference between Wade and the 'second banana' Cavs (Delonte West in 2008-09, Anderson Varejao in 2009-10) does little more than offset the difference between 'peak Cavs LeBron' and 'slightly past peak Heat LeBron'.
Chris Bosh, meanwhile, does not produce that much more than a bunch of other Heat players, at least not over the past two seasons. Hence, even though he is an All-Star, in terms of what he produces the wins produced metric does not rate him any higher than, say, Mo Williams on the Cavs (though Williams was named an All-Star too). And the rest of the squads are pretty even.
Have a look at the 2008-09 Cavs and the 2012-13 Heat, which both won 66 out of 82 games. LeBron and West on the Cavs produced as much as LeBron and Wade on the Heat. Williams, Varejao, Ben Wallace, and Wally Szczerbiak on the Cavs collectively produced a couple more wins than Ray Allen, Bosh, Shane Battier, and Mario Chalmers. But the Heat made up that difference through the rest of their squad, including Mike Miller and Udonis Haslem.
So, in summary, according to the wins produced metric, LeBron's Heat don't win heaps more than LeBron's Cavs because LeBron isn't quite as productive on the Heat as he was the last two seasons with the Cavs, Wade is clearly better than the Cavs' second best player but only by enough to offset the drop-off in LeBron's production, and Bosh is pretty much only an average NBA player on the Heat.