I mess with volumes all the time. I scale 5-gal recipes to 3, (so I can leave .25 gals of gunk in the kettle and .25 gals of trub in the fermenter and get 2.5 gals of good clean beer.) If I'm lagering something, I'll make sure to wind up with appx 3.25 gals final volume so it'll minimize the headspace in my 3ish gal lagering vessel.
Everything scales, and you can scale any recipe to whatever volume you'd like!