@Iluviaporos - Usually in cases where a large animal doesn't have many predators, they have a very long gestation period and don't produce many young over their lifetime. Since these were herbivores, they probably weren't actually able to run around all that much, as their bulk would have taken a lot of energy without them wasting it on activity. So their young might have been quite vulnerable to predators even if the adults weren't.
Also, it's hard for us to imagine the lifestyles of huge mammals now because there is so little wilderness left in the world. But they would have had millions of acres of food to eat and likely very little to stop them from roaming to find it. The only thing that would have kept mammals from growing even bigger would have been the effects of gravity on huge bodies and their systems, rather than because they might run out of food.