The effect of crystal size, pressure, temperature, and their coupling on the bandgap (E G) of ZnO crystals have been investigated based on the Hamiltonian perturbation, using the extended BOLS correlation theory. The functional dependence of the E G on the identities (order, nature, length, energy) of the representative bond for a specimen and the response of the bonding identities to the applied stimuli have been established. Theoretical reproduction of the measurements confirms that the E G expansion originates from the bond contraction/compression and bond strength gain due to (i) Goldschmidt-Pauling's rule of bond contraction induced by undercoordination, (ii) low-temperature enhanced stability, and (iii) mechanical energy storage. It is found that the multiple-field coupling effect dominates in the surface skin up to three atomic layers. The presented approach provides a guideline for harnessing the photoluminescence, photoabsorption, and exciton emission from ZnO and other semiconductors as well.