Is thicker Gai Lan less chewable and piercable? How does thickness of Gai Lan correlate with chewability? All 4 pictures of 薑汁生炒芥蘭 were taken at Tasting Court restaurant, Happy Valley, HK.
Thinner 1, 2
 
Thicker 3, 4
 
Thickest


Course #6: Stir-fried Kale with Ginger Essence
Good-ol-veggie dish with a touch of gingery kick. The restaurant peels the skin and trims the tops and bottoms of the each stalk in order to offer uniformity and the softest parts of the kale.
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.