Miami actually outscored San Antonio 42-40 in the second half... problem is they gave up 71 while "only" scoring 50 themselves in the first half. The 50 points at the half is a decent total, but clearly the 71 is "lacklustre" as fatman in de 10-button suit call it during the press conference. When they look at the tape that's what they'll see... not enough defensive tenacity in the first half.
None of that should take away from the Spurs offensive execution though... I can't stand them, but from an objective standpoint it's breathtaking to watch at times.