Parity-violating weak interactions produce extremely small energy differences between left- and right-handed chiral systems. We show that these microscopic effects may be amplified during collective phenomena such as phase transitions. The enhancement factor is proportional to the critical number of atoms, $N_c$, in the nucleus of the new phase. Measurement of the ratio of produced left- and right-handed chiral structures may provide a way to determine $N_c$. Experiments in which spin-chiral structures are formed in ZnCr${}_2$Se${}_4$ during a phase transition in crossed electric and magnetic fields indicate an effective amplification corresponding to $N_c\sim {10}^9-{10}^{10}$. We emphasize, however, that such large values cannot be inferred from homogeneous capillary nucleation alone, since the same parameters imply a very large nucleation barrier; in real crystals heterogeneous nucleation, defects, strain, and domain-growth dynamics may be essential. We also discuss, more cautiously, whether analogous collective enhancement could operate in first-order cosmological phase transitions beyond the Standard Model. The Standard-Model electroweak transition for the observed Higgs mass is a crossover and therefore has no critical bubble nucleus.