Dependence of the broken power-law spectral indices below and above a break frequency of 4 GHz (α₁ and α₂, respectively) on the source size, log D [kpc], for the subsets of the RxAGN dataset for different values of in log D [kpc], with corresponding errors in spectral indices. The error bars for log D [kpc] show the standard deviations of log D [kpc] within each bin. The numbers indicate the bin number in Table 1. The table lists the spectral indices radio SEDs shown in Fig. 6 for different subsets of RxAGN and correspond to numbered bins presented in Fig. 8.