Think of it this way in terms of personality / shyness.
Animals are evolved to be elusive and shy when they are tiny creatures that must fear for their life among even the smallest of predators. This generates very skittish animals that would routinely bite or musk if given the chance when handled or cornered. As we increase the size of the animals in question, more confidence is felt, and less fear stress comes into play. When dealing with Giant Snakes, you tend to see no human fear and thus more interactions that will take place. Retics and burms are typically not the snakes you would see hiding from it's owner or even a dog or cat that may be in the room. While many snakes will get scared of these situations - larger constrictors simply try and read the scene and get an understanding of their environment - you are part of that environment. This creates an entirely different personality than you would see in smaller pythons / colubrids. And makes them more tolerable to handling.
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