Without wishing to state the obvious, some synths sound bigger than others, and one of the over-arching reasons is the presence of more than one oscillator. You can do plenty with a single oscillator, but adding a second invites the opportunity to subtly detune it against the first. This slight jarring of pitch will have the effect of making a larger and thicker texture, all generated at the oscillator stage. For an even beefier sound, you can add a third oscillator; it is very common for a third oscillator to be placed an octave below the initial oscillator(s). Some synthesisers have this by default, normally being described as a sub oscillator.