This study comprehensively evaluates the bonding performance at the interface between circular hollow steel tubes and Ultra-High Toughness Cementitious Composites (UHTCC), by conducting a series of push-out tests on the composite structure, utilizing various lengths of steel tubes as a primary variable. Initially, the parameters of bond failure load and ultimate bond strength were explicitly defined, with the experimental values of the ultimate bond strength subsequently measured. In addition, in-depth research into the size effect on bond strength at this critical interface prompted the development of a theoretical formula for the size effect on ultimate bond strength, providing a new model for predicting these interactions. This newly developed formula was rigorously validated through a comprehensive error analysis, comparing theoretical predictions with actual experimental values, thus confirming the high precision and reliability of the model. These findings offer critical insights and have significant implications for both the analysis of interface bonding performance in hollow steel tube-UHTCC composite structures and the refinement of corresponding design methodologies.