Gradients in the stellar populations (SP) of galaxies – e.g. in age, metallicity, stellar initial mass function (IMF) – can result in gradients in the stellar-mass-to-light ratio, M_*/L. Such gradients imply that the distribution of the stellar mass and light is different. For old SPs, e.g. in early-type galaxies at z�∼ 0, the M_*/L gradients are weak if driven by variations in age and metallicity, but significantly larger if driven by the IMF. A gradient which has larger M_*/L in the centre increases the estimated total stellar mass (M_*) and reduces the scale which contains half this mass (R_e,*), compared to when the gradient is ignored. For the IMF gradients inferred from fitting MILES simple SP models to the H β, 〈Fe〉, [MgFe], and TiO_2SDSS absorption lines measured in spatially resolved spectra of early-type galaxies in the MaNGA survey, the fractional change in R_e,* can be significantly larger than that in M_*, especially when the light is more centrally concentrated. The R_e,*–M_* correlation which results from accounting for IMF gradients is offset to smaller sizes by 0.3�dex compared to when these gradients are ignored. Comparisons with ‘quiescent’ galaxies at higher z must account for evolution in SP gradients (especially age and IMF) and in the light profile before drawing conclusions about how R_e,* and M_* evolve. The implied merging between higher z and the present is less contrived if R_e,*/R_e at z�∼ 0 is closer to our IMF-driven gradient calibration than to unity.