Are you preloading animations? If the answer is yes, are you manually unloading them later on when you no longer need them? Preloaded animations stay in the cache until you manually unload them. The issue is likely not the RAM, but the VRAM - that's what is being used the most in terms of your graphics/animations, once you go past the available VRAM it will start eating into the RAM & will eventually start causing your game to lag.
Then again, it could be a mac issue. Does it generate a dmp file when it crashes & if so, have you sent it off to Simon? He will be able to tell you exactly why it is crashing. If I remember correctly the generated dmp file would be in the same folder that stores the save game files for your game.