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.
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invisibleInwardLightSpherePart.sh3f
(9438 bytes)
(Download count: 12849)
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Emmanuel Puybaret, Sweet Home 3D creator
