It is a combination of many thing done in a perticular way...well every song is different but i know what you mean.
Try adding slight reverb to some of your tracks, minimal on drums, can go wild on pads with reverb, always nice to have a little on the bass too!
Its about the hats/ shakers in the background, mixing in just right so you cant REALLY pick them out of the mix but they make and gel the whole drum loop.
In terms of actual sample choice / bass patch you have made. The fullness can come out of mathematically building your synth (layer samples) so it is detuned just right (opposite waves) the octave spread of your bass, many other elements too that change according to your DAW - for instance in Reason, on Malstrom, i can make sounds wide, narrow, short amp envelope, tonal, and a huge array of other synth settings that you can use to morph the sound and let it breath with the right compression settings.
Aswell, make sure there is always a new noise / fx / hit to grab your attention and draw you to it, making the whole track just just stand out and surprise....you may find it feels "fuller"
Also, get a sub sat under your bass.
And most importantly, experiment till you are nearly bleeding, get a bandage and do some more dial twisting