Mass–size relation. Top: stellar mass distribution function of the chosen sample of ETGs from observational data and the TNG simulations. Bottom: Circularized effective radii R_e, of the selected ETG samples in the TNG100 and TNG50 simulations as a function of stellar mass and compared with observations (ATLAS^3D and MaNGA). Both TNG50 (solid green line) and TNG100 (solid gray line) galaxies share comparable stellar masses and sizes with the ATLAS^3D (dashed red line) and MaNGA (dashed blue line) galaxies. The lines and points represent the median values, with colored regions and error bars indicating the 16th to 84th percentile range of each distribution. To avoid overlap, we shift the error bars slightly for clarity.