Re: I can't be imagining things, can I?
Oohh yeah now I get it. You're talking about the compression. When you turn on that switch, even if the volume is pedal is the 'same' you will notice that the initial attack sounds a bit quieter, because the sound itself is being squished. The distorted sound naturally compresses your tone a bit (or a lot, depends on the pedal), which means the tone evens out, the louds become a bit quieter and the quiets become a bit louder. Think of turning on a distortion pedal as like pouring water into a funnel, it starts wide open and it gets pushed into a focused stream as it comes out the other side. This subdues the highs and the bass a touch as well. Mind you, this isnt a bad thing at all, this is purely the nature of clipping and distortion. We ALL love the sound of distortion even if on a fundamental level it is 'destroying' the tone.
Now, as for why it sounds louder at the same time. Like I said with the funnel, it comes out as a much more focused sound, and the part that DOESN'T get squished as much is in the midrange. Guitar is a midrange instrument, and with the bass and treble being squished a bit, this causes that specific range (the range that IS the main voice of the guitar) to stand out. As a result, our ears actually pick it up as being louder overall, even if we carefully try to make it unity with the clean sound.
On an interesting note, and this might be different in your country, but have you ever noticed how LOUD advertisements are on TV? In most cases the TV company itself controls the volume of the broadcast, and would try to equalise the sound of the programmes and the advertisements. The advertisers used a bit of a dirty trick, and most television ads are HEAVILY compressed, focusing that midrange making it sound THAT MUCH LOUDER even though it's 'technically' at the same volume as the tv shows around it.
That all being said, things should get louder when you turn your drive on. If you set it for exact unity volume you will get the exact issue you are facing: It sounds weirdly quieter when you turn the pedal on because of all the compression. In your own words, turn it up so that the fundamental is LOUDER than your clean tone. It may sound weird to have a bigger jump in volume when practicing on your own, but in a band setting it'll fit in much better in the mix.
