FabFilter

The "default" preset has a threshold & ratio set as well as autogain enabled. Are you sure it doesn't do any compression? The "clean" preset (same, but has a 1:1 ratio and therefor doesn't do any compression) completely nulls with the unprocessed audio, so no increase in volume.

Thanks for the answer.
I'm quite sure there is no compression, the audio is not hitting the threshold.
I understand that autogain is enabled. But if the compression is cutting like 1db I should have 1 db of again from the autogain function, but I have more than that 1db with kontakt instrument not hitting the threshold.
In any case I simply write a midi track, I open Pro-c2 and I have an increase in the volume even without compression.
I tested with a Macbook and an Imac and it happens the same.
I can make a video of it but not sure I can upload that in here.

Thats' the autogain that's on. Turn it off and the volume rise will vanish. Also keep in mind that the default preset has a soft knee, so it begins before the audio peaks hit the threshold.
I think the autogain is calculated based on the settings of the compressor. So even if you don't have any gain reduction from the compressor going on, it will adjust the gain. This makes sense, let's say the auto gain would only work when you have gain reduction, that would mean the output gain would change all the time - not very desirable.
Hope this helps.