For nearly two decades, research groups have attempted to increase the thermal conductivity of bulk materials by saturating them with different concentrations and types of nanoparticles. However, optimal enhancements in the thermal conductivity of these materials continue to go unrealized, primarily due to interfacial phenomena that impede phonon propagation from the bulk material to the nanoparticle or between individual, contacting nanoparticles. Though it is almost certain that interfacial resistances between contacting nanoparticles are responsible for the underwhelming thermal performance of nanoparticles in bulk materials, the physical mechanisms that limit thermal transport at nanoparticle junctions is not well understood. In this study, we investigate the effect of nanoparticle constriction size (i.e. the contact area between linked nanoparticles) on the bulk thermal conductivity of a surrounding paraffin-based phase change material. To this end, we measure the effective thermal conductivity of different Multi-walled Carbon Nanotube (MWCNT)/paraffin nanocomposites is measured with the transient plane source technique. Using a newly developed physical model, the interfacial thermal resistance between the contacting nanoparticles is determined as a function of the constriction geometry. Results suggest that the thermal conductance (i.e. the rate of heat transfer) between contacting MWCNTs can be increased by a factor of 27 by increasing their diameter by only 58nm. This is in direct conflict with effective medium approximations, which predict an increase in the bulk thermal conductivity of MWCNT–PCM composites when MWCNT diameters are reduced. Such a result indicates that the contact area between individual, contacting nanoparticles has a significant impact on thermal transport within the PCM nanocomposite, and therefore its bulk thermal conductivity. It is expected that these results will strongly impact the design of nanoparticle-laden PCMs.