The average mean‐square radii of gyration for the products of the linear polymerization, the star‐shaped polymerization, and the hyperbranched polymerization of AB2‐type monomer in the absence or presence of a multifunctional core initiator are investigated using the Dobson‐Gordon's method. The dependence relationships between the average radii of gyration and the average degree of polymerization calculated using the Dobson‐Gordon's method for the linear polymerization products are in good agreement with those obtained from the matrix algebra method of the rotational isomeric state model. The radii of gyration of the star‐shaped and hyperbranched polymers are much smaller than those of the linear polymers if their average degrees of polymerization are equal. The conversion of groups, the core/monomer feed ratio, and the core functionality affect the average radius of gyration of the products. However, compared with the other factors, the degree of polymerization is the most influential factor on the average radii of gyration for the hyperbranched polymer system.