Is thicker Gai Lan less chewable and piercable?
Simple answer, the thicker the vegetable the older it is. The older it is the tougher and especially more fibrous, the skin and the stalk in general is. You'll see the same with any green stalky vegetable (eg: Broccoli). It's peeled especially with the older plant, as if it's old enough it could be quite fibrous. It looks like the first picture isn't peeled. Likely because it's young enough, that the skin is still very tender.