There are a couple reasons her shins might appear fat. Firstly, the fat has to go somewhere and it's just part of being morbidly obese. Secondly, I injured my ankle long ago, and if I'm up on my leg without resting 12 hours, my leg blows up to the knee on the affected side. Maybe being 600 lbs is like having an old injury like that.
Also, one reason you will never see Amberlynn's legs is the varicose veins, hair, and everything else gross about them. It's not just that she's weird about her big damp tree trunks.