Abstract: Based on the change rate of the gale-force wind radius (R17) of tropical cyclones (TCs) over the Western North Pacific, the evolution of TCs is categorized into size rapid expansion (RE) and size rapid contraction (RC) events. This study explored the evolution characteristics of precipitation cloud systems in the outer core regions of TC in RE and RC events as well as the physical mechanism responsible for the R17 change. The satellite data of Tropical Rainfall Measurement Missions and Global Precipitation Measurement, the International Best Track Archive for Climate Stewardship dataset, and Multiplatform Tropical Cyclone Surface Winds Analysis from 2007 to 2016 were employed. The results demonstrate that (1) the precipitation intensity in the TC outer core region of RE events is significantly higher than that in the RC events, showing that the TC outer core region's precipitation intensity plays a key role in the growth of R17. The heavy precipitation distribution in the initial stage of RE events is more scattered than that in RC events, which can serve as a precursory signal to differentiate the two kinds of events. (2) The common characteristics of precipitation in the outer core region of TC in the RE and RC events include stratiform (convective) precipitation with slow (high) precipitation intensity and large (small) precipitation area and dominant diabatic heating in the middle and upper (middle and lower) atmosphere. Convective and stratiform precipitation have comparable standardized precipitation rates. However, the precipitation area, precipitation intensity, and standardized precipitation rate in the outer core region of RE events are higher than those in RC events. (3) In RE occurrences, the outer core region of the TC exhibits high inertial stability and strong diabatic heating, resulting in a high kinetic energy wind field that is conducive to TC size expansion. Additionally, the increase in kinetic energy encourages the growth of low-level input, which aids in the expansion of R17 and the formation of convective cells.