Organic semiconductor (OSC) nanoparticles (NPs) are promising for numerous applications including greener organic photovoltaics and heterogeneous photocatalysts for solar H2 production. Single component or mixed bulk-heterojunction (BHJ) OSC NPs are commonly prepared from conventional polymer OSCs via the miniemulsion-evaporation method using ultrasonication. However, realizing the expected NP size control with this approach remains elusive, limiting optimization. Here, we demonstrate that the presumed miniemulsion-evaporation mechanism is not the principal pathway forming NPs. Predominantly, a direct extraction of OSCs from the organic to the aqueous phase during ultrasonication results in NP formation prior to organic solvent evaporation, rendering NP size insensitive to emulsion parameters. By replacing ultrasonication with lower-energy shear mixing, we control the competition between these pathways, achieving tunable NP sizes via a true emulsion-evaporation mechanism. This enables the first demonstration of BHJ NP size effects on photocatalytic H2 evolution, with a similar to 2-fold increase in H2 production when reducing NP diameter from 230 to 160 nm. However, the observed similar to 14-fold higher performance of direct-extraction BHJ NPs (25 nm diameter) highlights the need to reassess OSC NP formation. Overall, this work advances an understanding of photocatalytic activity via size optimization and offers a greener processing route by eliminating organic solvent evaporation.