Considering we use our dominant arm for lifting, most tasks, holding things, etc.
In: Biology
[Your forearm muscles pull tendons to move the fingers](https://assets.answersingenesis.org/img/articles/am/v5/n3/index-finger-muscles-large.gif); this way you can have a lot of muscles to grip strongly, without them needing to be located IN your hand / IN your fingers. So for something like a sword-and-shield medieval fighter, where they grip the sword tightly with their right hand but “lift” the shield with the entire left arm, you would see a thicker right forearm.
Otherwise, with day to day activities we use our dominant hand for “precision” stuff like writing and fine manipulation of objects, and precision doesn’t require muscles.
Heavy things that do require muscles, we tend to lift with both hands, so your biceps tend to develop evenly.
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