McCarthy was deemed too large for a Premier League player, especially for a club fighting relegation, and was ordered to work on his fitness. Failure to meet his target would result in hefty financial penalties.
The former SA national team player did meet his target but was left out of the 25-man West Ham squad for the second half of the season.
Hammers boss Avram Grant believes the weight loss has slowed the striker. "He reached the targets but I think that affected his movement," said Grant.
"He's a very good player and one of the best finishers in training that I have ever seen, and I have had some good players. But now we need players with movement and the advantage of being quick, and we have others."
McCarthy was challenged at the beginning of the season to lose 2.2lb in weight for five or six weeks or face a fine of one week's wages each time he failed.
He reached his target of 13st 5lb by October after ballooning to more than 15st at one stage and his body fat went down from 24.2 percent at the end of last season to 16 per cent by mid-August. A professional athlete is expected to carry from five to 12 percent.
Meanwhile, his agent Rob Moore, told the Daily Mirror that McCarthy wanted to play and did not want to just sit around and collect his cheque. McCarthy earns £38 000 a week and appears to have been fined five times in his long-term weight battle.
''Avram has a job to do and the team are in a difficult position, so you can't blame him for making decisions he believes in, but Benni has had hardly any game time and it's inevitable he would get a bit disheartened.
''He's at the level of fitness and weight the club prescribed and they are happy with that. But he is 33 years old and we have to be realistic," Moore told Daily Mirror.