The nature of an apparently random rock deposit, such as regolith on an extraterrestrial body, is often evaluated in terms of the size distribution of the rock particles. Especially, the power-law index of the cumulative size-frequency distribution (CSFD), , is often used to describe the distribution. However, an value is typically obtained only from surface images, raising questions about its ability to reflect the size distribution of the entire deposit. In this study, we perform laboratory and numerical experiments to demonstrate that an value obtained from surface images alone does not necessarily represent that of the entire granular deposit. The value is only a reliable indicator of the size distribution of surface rocks, which can still be useful to gain insight into the evolutionary history of an asteroid’s surface. However, to fully address the internal information, direct observations of subsurface materials or sample analysis are required.