This study (Adv. Mater. 2024, 2404054) elucidated the overlooked correlation between particle size and energy efficiency in polymer photocatalysts. Here, a dispersion polymerization technique was developed to control the size of conjugated microporous polymer (CMP) nanoparticles from 15 nm to 180 nm. By analyzing the relationship between the particle size and photocatalytic reactivity through modeling and femtosecond time-resolved spectroscopy, the study revealed that the accessibility of substrates to excitons with short lifetimes significantly impacts the energy efficiency of polymer photocatalysts. Consequently, CMP nanoparticles synthesized based on these design principles achieved over 10-fold improvement in energy efficiency photocatalytic reactions compared to conventional CMPs.