The human body is a marvel. Different parts of your body can be strengthened enormously through training. You can bet that if we cut off his hand and your hand, defleshed them and took a look at his bones and tendons they would look positively enormous compared to yours.
Basic physiognomy / genetics probably plays a foundational role too of course. We can probably assume that the kind of person who builds toward that particular sort of stunt probably had pretty strong fingers to begin with, to even get that idea.
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