Proprietary model. ;)
jk. You can show that the radix join's nested loop equi join has a mean duration that can be modeled by a/x+b where x is the number of instances and a and b are dependent on the table size of both inputs combined. This is the result of a curve fitting (least squares regression). It it specific to the gaza server. We decided to codify this server specific model anyway since it still holds a monotonicity property if run on a different machine: i.e., even if the exact numbers are of, if you requested a smaller theoretical mean task size, you will get a smaller actual mean task size compared to a bigger mean task size. This explanation might be a little too compressed. If you want to have a more thorough explanation, let me know.