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The quantities assigned as half light radii appear to come from two sources. The half light radii of bulges appear to be assigned using the galaxy formation models of Baugh et al (2005; MNRAS, 356, 1191). To arrive at this conclusion, I queried 27,000 galaxies from our database and divided them into bins based on the absolute magnitude of the bulge. Consider the plots below, which show the distribution of "half light radii" (as defined the database) in each of these bins:
Comparing these distributions to the blue curves in the "early type" panels of Figure 15 from Gonzalez et al. (2009; MNRAS 397, 1254), we see distinct similarity. Given that the blue curves in that figure represent Petrosian half light radii drawn from the Baugh 2005 model, it seems reasonable to conclude that the half light radii assigned to galaxy bulges in our database come from the same model.
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