My research project is focused on morphologically peculiar galaxies in IllustrisTNG50-1. I'm using several supplementary catalogues to compare properties of selected simulated galaxies with their real-life counterparts found in observations.
Working with supplementary catalogues (e) Stellar Projected Sizes based on modelling presented in Genel+ (2018) I've found that stellar half-mass radii presented in that catalogue does not match those reported in Subhalo catalogue. I believe this difference comes from "projection effects", although there are no details on how the "projected half-mass radius" is calculated in the paper (to my understanding, it is a direction independent value). But I'm probably missing something, hopefully you could clarify this discrepancy.
There are many different size measurements in the "(e) Stellar Projected Sizes" catalog, and you are correct that all of these are different from values you can find in the subhalo catalogs, the most basic of which is the stellar half mass radius (in 3D).
The calculations in that catalog are described in the referenced paper, namely the half-light radii. For example: "circularized projected half-light
sizes... sizes that contain half the optical light in the r-band... two- dimensional (namely, based on projections along a random line of sight); and are circularized (namely, averaged between the extents along different directions on the observed plane)."
Thanks a lot for your answer!