Abstract: In recent years, the study of exoplanets has become a hot topic in astronomy and planetary science. With the rapid development of detection technology, mankind has discovered and confirmed thousands of exoplanets, and has carried out in-depth studies on the atmospheric composition, orbital properties, habitability and other key characteristics of these exoplanets. In this context, star-planet atmosphere interactions have been recognized as one of the central mechanisms affecting the evolution of planetary atmospheres and their habitability. In this study, we aim to reveal the role of planetary radii in the long-term evolution of their atmospheric escape. By building a three-dimensional magnetohydrodynamic (MHD) model and combining it with the evolutionary characteristics of the stellar system, we reveal the role of planetary radii in the long-term evolution, and construct a numerical simulation system based on the Venus-like atmospheric escape model, taking Kepler-1649 c and its host star as the research objects, which covers different planetary radii and stellar ages. It is found that the planetary radius significantly changes the escape contribution ratio of each ion. Among them, the escape rate of O⁺ as a proportion of the total escape rate decreases with increasing planetary radius, from 99.3% to 17.1% at 4.8 Gyr. Meanwhile, the variability between the O⁺ ion distribution and the total ion distribution in interplanetary space increases with radius. This study provides a new perspective for understanding the mechanism of exoplanet atmospheric evolution by discovering the differential effect of planetary radius on the escape behavior of different ions.