It was a generalization, like many others expressed here. I agree that 6 feet may be an extreme indicator; over 9 or so is much more of a concern. Good point.
How many larger snakes have I had? I've had several. Eastern Kingsnake, Gopher, Burmese Python, Rat Snakes. When my 3-foot Burmese began hissing and challenigng me on the floor, I knew that I had made a bad mistake. Same thing with Boa Constrictors, which can exceed 12 feet. (Have you ever seen the picture of the one that ate an oil field worker in Venezuela?)
Reality time: Your pythons eat the same food as your Kingsnakes - only becasue you FORCE them to. Like kids being forced to eat hamburgers, fries and carbonated sugar water.
But them, how can I expect the average American to understand complex carbohydrates, fiber and enzymes? 66% are overweight; 31% are so overweight that it affects their health; 33% of our women can no longer give birth naturally without surgery. Let's not forget about how popular Viagra is - as opposed to making diet and lifestuyle changes which would have cured impotence naturally. Most Americans value convenience more than wisdom; it's reflected in what they eat - and feed their pets.
Most boas & pythons are primarily bird, bat & lizard eaters. Bats, parrots (locale-specific) and such. They rarely encounter house mice in the wild. Most snakes prefer wild mice over house mice; they refuse house mice when they have a choice of deer mice, pocket gophers and other wild prey.
Try feeding your snakes ONLY the feed that they would have gotten in their wild habitat - live & fresh - then see if they ever take your frozen/thawed house mice again!