Size-dependent properties define a hallmark characteristic of nanomaterials, enabling scientists and engineers to create materials with tunable properties. Yet, these size-dependent properties in a polydisperse ensemble of nanomaterials results in a distribution of properties, which is often undesirable. Realizing the promise of nanoparticle-based technologies demands more efficient, robust synthesis methods (i.e., process intensification) that consistently produce high-quality and large-quantities of nanoparticles (NPs). Traditionally, NPs are synthesized via a colloidal hot-injection reaction, in which an organic-phase reagent is rapidly injected and mixed at high temperatures to burst nucleate NPs. The temporally short and fast nature of the hot-injection method introduce stringent demands on the mixing time-scales of the reaction, a critical factor in the scaling up of nanomaterials. To address this issue, we employ the heat-up method, whereby the organic-phase precursors are mixed near room temperature and then heated past the nucleation threshold temperature. The low temperature mixing ensures mass and thermal uniformity of the reagents, reducing processing constraints. We study the NP synthesis via a heat-up method in a regime of previously unexplored high concentrations near the solubility limit of the precursors. We seek to answer the central question, how does the nano-synthetic chemistry of the heat-up method differ when concentrations are intensified? In this highly concentrated and viscous regime, we discover the NP synthesis parameters are less sensitive to experimental variability and thereby provide a robust, scalable, and size-focusing NP synthesis. Mechanistically, our investigation of the thermal and rheological properties of the colloidal mixture reveals that this high concentration regime has an approximate order of magnitude increase in solution viscosity and heat capacity, therefore reducing mass diffusion, stabilizing the reaction to perturbations, and deterring the onset of Ostwald-ripening. Compared to the conventional synthesis methods (hot-injection with dilute precursor concentration) characterized by rapid growth and low yield, we synthesize high-quality metal sulfide NPs, and demonstrate 10-1000 fold increase in NP production relative to the current field of large-scale and lab-scale efforts. The controlled growth, high yield, and robust nature of highly concentrated solutions can facilitate large-scale nano-manufacturing of NPs by relaxing synthesis requirements to achieve monodisperse products.