And when the Heat play Lebron at PF, they are not going small. They are going fast.
Or they're just using a lineup that allows them to put their best players on the floor. Which means 3 wings, LeBron, and Bosh.
Lebron is close in size, stronger, and much faster and more explosive than almost all PFs in the league. Most PFs are under 6'10 these days, with quite a few around 6'8 or 9.
That's why Lebron can play PF; it might be a bigger advantage than having him at SF.