Tl; dr version: Leonardo da Vinci rediscovered Vetruvius’s theory of proportions. Early strongmen then noticed parallels from Renaissance statues to how they trained their bodies.
The Golden Ratio.
I took most of my ideas from this site, though the idea is fairly widespread
How it practically looks:
- Flexed arms 150% larger than the circumference of your non-dominant wrist.
- Flexed calves should match flexed arms.
- Your shoulders should be 1.618 times your waist.
- Upper leg should be 75% larger than knee circumference.