caption: Stellar mass surface density correlation with the size of the detected stellar structures. As in the mass--size relation, the strength of the correlation between density and size changes significantly from one group of structures to the other. No significant relationship between these parameters exists for the low-density complexes (blue and purple symbols), for which density is almost constant. In contrast, the correlation of surface density and size becomes progressively stronger for systems detected at higher density levels, with the most compact systems showing the most prominent relationship between these two parameters (red and orange symbols).