I think it’s mainly due to the fact that 's scene is composed with very simple objects and a very limited number of texures (used in a great way, I add)
A big part of the size is probably due to the DerivedDataCache embedded in the realistic rendering project (which, if I understand correctly does not necessarily need to be bundled with the project.
If you are talking about the UV layout, is probably to add more density in places where it is needed. From a first glance to the project, the lightmap density is more or less the same all across the scene (= the texel size is always more or less the same, and the lightmap size is change accordingly to achieve )