Predicted by the the promising hierarchical formation model, the extended stellar halos of massive galaxies keep crucial fossil records of their assembly history. However, due to the low surface brightness of these halos, much of these information are yet to be understood. With the help of deep i-band images of a large sample of massive central galaxies at 0.3 < z < 0.5 from the Subaru Hyper Suprime-Cam (HSC) survey, we map their stellar mass distributions out to radius larger than 100 kpc individually. We find that the stellar halos of massive galaxies show a larger intrinsic diversity than their inner structures. Both the slopes of the surface brightness profiles and the shapes of the stellar halos depend on total stellar mass where more massive galaxy tends to host more extended halo with shallower slope and more elongated shape. Moreover, we also discover subtle, but systematic and robust structural differences that depend on halo mass. Massive central galaxies from more massive halos show significant excess of mass in the outskirt. These differences translate into an “environmental” dependence of the mass–size relation. Such differences are broadly consistent with richer recent merging history for more massive halos. These results also highlight the importance of deep photometry and the usage of detailed structural information in the study of the assembly of massive galaxies. Applications of this dataset in studying the galaxy-halo connection at very high-mass end will also be discussed.