I think that if there is absolutely no information on the tracks of a synth, "Freeze Synth" should not generate any audio file, right?
But even with all tracks empty, when I use "Freeze Synth", Sonar generates a huge WAV file!
Does anyone know how to fix this?

What are "the tracks of a synth?" 

What soft synth are you using?, so I can try and replicate.

rogeriodec
But even with all tracks empty, when I use "Freeze Synth", Sonar generates a huge WAV file!
Does anyone know how to fix this?

Delete the empty tracks before freezing the synth or at least set the track inputs to none on the empty tracks.

slartabartfast
What are "the tracks of a synth?" 

The midi tracks that are associated to a Synth. 
For example, I have 4 midi tracks linked to an instance of Kontakt. All tracks are empty. I click "Freeze Synth" and a WAV file is created (bounced) in the audio folder. I think this file should exist only if there is at least one note in any track. But not when tracks are empty.

scook
Delete the empty tracks before freezing the synth or at least set the track inputs to none on the empty tracks.

This is not very friendly, right?

What does the wave file that was generated sound like?
 

Grem
What does the wave file that was generated sound like?
 

 
A 360mb WAV file, 44.100 Hz, 32 bit (float), completely mute.

rogeriodec

scook
Delete the empty tracks before freezing the synth or at least set the track inputs to none on the empty tracks.

This is not very friendly, right?

Keep in mind, you told SONAR where the synth output should go. You decided not to use the tracks. Then you told SONAR to freeze the synth. Now you want SONAR to remove empty waves after you instructed it to generate them. I suppose there could be another freeze option but it definitely would be a feature request. SONAR would have no idea in advance of freezing if there would be audio generated or if the empty tracks were created intentionally so the delete would happen after the freeze.

I believe that a mute audio file will be equal in size to a loud audio file at same length.