on a more serious note, we cant really tell if a cat is fat or skinny by picture. But if TS has a real reason to be worried, have you deworm your cat?

Eating healthily but still being skinny points to worm issues quite likely. Anything beyond this are usually more complicated maters which are usually not.

Healthy cat should not be boney, they should be firm to the touch and not too boney and muscle/lean meat can be felt across the spine and the spine usually doesnt feel sharp(like a jigsaw surface).