The relationship between glScalef() and light intensity

I used to call glScalef(…) to zoom in/out the scene. But I found that the light brightness will be reduced if I zoom in the scene by increase the glScale(…) parameters. And I am sure that other gl parameters are unchanged while glScale paramters increasing.
Can you tell me the relationship between brightness and scale? Thanks.

the normals are also scaled.
this is answered in the faq rightside of page

Use glEnable(GL_NORMALIZE);