Sorghum genotypes vary in their response to higher sowing density, but the traits explaining these variations are unknown. In the present study, a 3D-imaging based approach identified the phenotypic traits responsible for the genetic variation in sorghum's response to high sowing density. Twenty sorghum genotypes, some varying in their response to density, were grown and 3D-images were collected weekly between weeks 4–6. From these images, 80 phenotypic traits, including 33 architectural and 47 multispectral, were extracted. The within-genotype means of these 80 traits, and two indicators of the sowing density response (Biomass ratio (Br) and Transpiration ratio (Tr)), measured in a previous study with 13 common genotypes, were used in a Spearman correlation analysis. Seventeen and four traits were strongly correlated with Br and Tr, respectively. The majority of these traits, predominantly architectural, strongly suggest that, under high sowing density, a fuller light interception, having more leaf area in the lower canopy, lead to a larger Br, while more vertically aligned leaves favour larger Tr values, which related to higher water use efficiency in another study. Furthermore, a Principal Component Analysis (PCA) indicated traits contributing to better photosynthesis could be used to estimate Br. Similarly, a combination of traits relating to leaf angle were good indicators of the genetic variation in Tr values. These results provide insights about the strategies some sorghum genotypes have developed to thrive under higher sowing density.