The East China Sea (ECS) is the most important fishing ground in China. Field observations have always found no positive correlation between phytoplankton stocks and zooplankton biomass in the ESC. It has been suggested that under eutrophic conditions, the phytoplankton biomass is too high to limit mesozooplankton growth and lower the energy transfer efficiency from phytoplankton to mesozooplankton. However, this hypothesis does not explain the higher zooplankton biomass supported by lower chlorophyll a in offshore waters. This situation can be referred to as a “mismatch between phytoplankton and mesozooplankton” or “variation of Z/P”. In this study, we used field data covering four seasons to test four hypotheses that (a) flatter slope of normalized biomass size spectrum (NBSS) of mesozooplankton enhances the Z/P; (b) higher mesozooplankton size diversity enhances the Z/P; (c) stronger predatory pressure from zooplanktivorous fish decreases the Z/P; and (d) larger-sized primary producers enhance the Z/P. The results of our study confirm that the size diversity of zooplankton was the most important determinant of spatial variation of Z/P in the ECS in each season except winter. The NBSS slope of zooplankton was also an important factor, but only significant in summer. Other potential mechanisms, including predation pressure from zooplanktivorous fish and larger-sized primary producers, were not determinants. This seasonal change was possibly due to seasonal variations of zooplankton community structure and functional structure in ECS, especially the large-sized and omnivorous Calanus sinicus transported by the stronger ECS Coastal Current during the winter. Our findings highlight the importance of a size diversity approach in the evaluation of the carrying capacity for fish in coastal ecosystems with conspicuous temporal and spatial variations.