The three axes physique judges read
Physique judging rewards more than raw size. Symmetry refers to left-right balance, meaning matched development between the two sides of the body. Proportion concerns the relationship between body parts, classically the relationship of shoulders to a tight waist and the balance of upper to lower body. Balance, sometimes called the absence of weak points, means no single muscle group lags conspicuously behind the rest.
Together these axes explain why a smaller but harmoniously developed physique can place above a larger competitor whose package is uneven.
How these ideals trace back to classical aesthetics
The proportion ideal in bodybuilding draws on older notions of the classical figure, including the small-waist, broad-shoulder taper associated with the so-called golden era. Athletes use this framework to identify which body parts to prioritize so the whole physique reads as cohesive from every mandatory angle.
Judging standards differ by federation and division and are partly subjective, so what counts as ideal proportion varies with the rules of the specific competition.