Transition metal carbides are increasingly used as catalysts for the transformation of CO2 into useful chemicals. Recently, the effect of nanostructuring of such carbides has started to gain relevance in tailoring their catalytic capabilities. Catalytic materials based on molybdenum carbide nanoparticles (MoCy) have shown a remarkable ability to bind CO2 at room temperature and to hydrogenate it into oxygenates or light alkanes. However, the involved chemistry is largely unknown. In the present work, a systematic computational study is presented aiming to elucidate the chemistry behind the bonding of CO2 with a representative set of MoCy nanoparticles of increasing size, including stoichiometric and non-stoichiometric cases. The obtained results provide clear trends to tune the catalytic activity of these systems and to move towards more efficient CO2 transformation processes.