The major reason is that today men has various comforts to live his life without any difficulty. He don't have to struggle more as the animals have to . Animals have some many difficulties like it can't find food on time as we people get. You might notice that an overweight weight animal is that which is being cared by his master and gets proper food on time without any struggle. So that is why population of overweight people is more than overweight animal. 
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