Answer:
Shmuel is starving and Bruno is well fed. That is the reason why their fingers are so different. Shmuel lives in a death camp while Bruno lives in a house with food and servants.
Explanation:
I don't exactly remember the page in this book where this is from, but good evidence would be when Bruno's father found Shmuel eating bread Bruno gave him causing him to get beat by a soldier. Bruno felt bad because Shmuel looked so sick.