Why do extremely skinny people have much exposure of the eyelid platform?

I think your question basically boils down to: do thin people have less fat in their orbits (eye sockets), and the answer is generally, yes. As we get older, we lose the support and tightness of our skin and muscles in the face, which allows fat to drop, get exposed, or reposition vs. where the fat ideally sits. Or it just leads to uncovering of the hollow of the eye socket as the cheek fat descends, leading to visibility of the lid-cheek junction. For this reason, we usually do not remove a lot of fat from the lids anymore, but rather either supplement it to make it look more youthful (think about it: babies and young adults have a lot of eyelid fat), or reposition it to blend these transition points. However, if someone is truly anorexic or has massively low weight, overall fat composition everywhere will be lower.

Eating disorders with severe weight loss may cause deepening of the superior sulcus (area above the crease of the lid). There are also hereditary factors and anatomic variations. It is very difficult - maybe impossible - to predict how the eyelid area would change with weight gain. Please AVOID fat or filler injection directly into the eyelids in an attempt to fill volume as this can cause severe problems with ptosis, diplopia, and a lumpy appearance - unintended consequences that are very difficult to surgically correct. Best wishes.