Got them 2 years ago as kittens, both were identical orange tigers. Since then, they've become totally different from one another both physically and personality wise. One is slightly underweight, very adventurous, loves attention (constant head bunts), very vocal, but oddly doesn't purr. Brother is a fatty, jumps whenever he hears a noise (a cowardly lion kinda guy), doesn't care for a lot of human interaction, only makes a squawking noise when he tries to meow, and purrs like a motorboat. They both have equal access to food, so it's not like fatty is stealing the little guy's food, but he's nearly twice as heavy as his brother. Also concerned about little guy not purring, you can hear it if you hold your ear to his chest, but otherwise it's inaudible. Is this normal?