Another explanation is that the size of the thermal pad on the bottom of the package doesn’t have much affect on anything, because something else in the chain is the limiting factor.
The area of smallest contact in the chain is between the bottom of the die and the top of the package (substrate). That area of contact is the same between XPL & XML2 because the die is the same size. The area of the thermal pad on the bottom of the package is larger and therefore carries less heat load per unit of area. The larger the area, the poorer the thermal performance (C/W) can be to get equivalent performance.