pimping tracks on a DAW before playing?

i work in a club and have noticed that some DJs who happen to be producers as well, have the waves of the tracks to the fullest (they reach top to bottom on the CDj screen), even on tracks that are not theirs.
This doesnt match my experience buying tracks (in any format), many of them have waves that are just above half the space available... how is this possible?
My guess is that they tune the tracks up in a DAW before playing, with a saturator or limiter... is that whats going on? is there more to it? am i crazy?

the roots of my questions lays in my negative experience mixing tracks that have uneven waves (volume?) and i wanna deal with it the best way possible

tips appreciated.

PS: i have decent knowledge on ableton