Here is another reference to humans....... If a person "overfeeds" and "reaches its full potential too fast" this is called obesity. This is the same thing in snakes.. When a snake is overfed, it gains the size through fat, not muscle. This results in an unhealthy snake and a shorter lifespan, just like in humans. If a snake is fed the correct amount, it builds more muscle and does not have the fat issues, so it is therefor an overall healthier snake.
And the whole protien thing is a totally seperate issue. Too much protien in a diet without other supplements to balance the protien out can lead to a totally different range of health issues... The same reason that they say you cant live off of rabbits if you are stranded in the wild because of their high muscle content (which means very high protien in their meat) and very low fat content.