Why do Shmuel's fingers look different than Bruno’s?

From the text:

The two boys looked down at the same time and the difference was easy to see. Although Bruno was small for his age, and
certainly not fat, his hand appeared healthy and full of life. The veins weren't visible through the skin, the fingers weren't little more than dying twigs. Shmuel's (page 168) hand, however, told a very different story.