I don't know if Blender is able to optimize the vertices used in two different places when it exports to OBJ file but it's probably the reason why your model grew.
I tried on my side to optimize the "v" vertice lines in the OBJ file and got a smaller file.
----------------------------------------
invisibleInwardLightSpherePart.sh3f
(9438 bytes)
(Download count: 26101)
----------------------------------------
Emmanuel Puybaret, Sweet Home 3D creator
