I have two immediate thoughts about tattoos and fatties. First, that the tattoos hurt less because they aren't directly over the sensitive nerve endings around bone depending on where it is. Second, that the tattoos hurt less because the increased surface area of their skin limits the amount of nerve endings their body is capable of producing. For the second thought, it is well documented that many areas of the body are more sensitive than others. Not to be crass, but think of the difference about how a stroke of someone's finger feels around your genitals vs on your shoulder. While both will elicit a warm fuzzy feeling, the genital stimulation will be much more so. This has to do with the density or concentration of nerve endings. As such, that's why I think as their body expands, their nerves are not as concentrated in the skin.