The structure of mixed diblock co-polymer micelles in the aprotic ionic liquid I-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM] [TFSI], was investigated by dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and small-angle neutron scattering. The diblock co-polymers are poly(methyl methacrylate)-blockpoly(n-butyl methacrylate) (PMMA-b-PnBMA), where PMMA is solvent selective and forms the corona of the micelles, whereas PnBMA segregates into the micelle core. In these experiments, a pair of diblocks with the same corona block length but distinct core block lengths (N-core) was first mixed homogeneously and then allowed to self-assemble into well-defined mixed micelles. DLS and SAXS measurements show that the hydrodynamic radius (R-h) and core radius (R-c) increase monotonically with the fraction of the longer diblock in the mixture. Interestingly, the dimensions of the binary mixed micelles are significantly larger than those formed by a single diblock with the same number average core block length (< N-core >). Similarly, the aggregation number is also much larger for the mixed micelles. These results can be understood by recognizing that in the binary mixed micelles, the shorter core blocks are not necessarily stretched to the center of the micelle core, which will be predominantly occupied by the longer core blocks. This relief of the shorter core block stretching effectively increases the aggregation number. Free-energy calculations confirm this hypothesis and are able to predict the R-c values of the binary mixed micelles by eliminating the elastic free energy of the shorter core blocks. These results provide new physical insight into the micellization of binary co-polymer mixtures and demonstrate that block polymer blending is a facile strategy for precise tuning of the micellar structure (e.g., core radius and corona density).