Membrane technology without a phase-change process has demonstrated its low-cost, high-energy-efficiency advantages. High fractional free volume polymers of intrinsic microporosity (PIMs), such as poly(1-trimethylsilyl-1-propyne) (PTMSP), are known for high permeability but disappointing gas selectivity. Porous aromatic frameworks and hyper-cross-linked polymers (HCPs) are effective membrane additives; however, prior works have examined their performance through chemical interactions and other material characteristics. Using a recently developed facile and scalable controlled HCP synthesis method, the size effect of these additives can now be investigated. In this work, HCPs with gradually increasing molecular weight/particle size were combined to investigate the performance and mixing/dispersion behavior in PTMSP mixed-matrix membranes. Unexpectedly, the largest dispersible additive (V125E) resulted in a membrane with drastically higher selectivity (H2/CH4 = +690%) and similar H2 permeability, which is a performance above the 2015 upper bound for H2 gas pairs. With identical additive affinity/compatibility, we demonstrate the benefit of size-controlled HCP additives for enhancing membrane performance.