This is for mitigating the recently discovered problem with attenuated lights getting reduced in size, even on OpenGL 3+. The intent of the shrinking was to account for higher brightness of non-attenuated lights on OpenGL 2 and was never meant to be active on more modern versions.
The factor will apply to any attenuated light defined after it and will be inherited by included sub-lumps, but it will only last for the lunp it is set in.
If you have a definition for the broken behavior, AddLightAssociation
'lightsizefactor 0.667' at the top of your GLDEFS.