It depends heavily on the exact usage: http://dictionary.reference.com/browse/quantum?s=t

The salient feature here is that quanta are discrete and a transition from one to another is a jump rather than a smooth or continuous shift.

It's a Latin noun, derived from an adjective, that literally means "a something of some size." That's it. The first meaning in English is "some quantity." Uses which denote or connote 'large' or 'small' are secondary extensions which depend on context.

"Quantum leap" connotes an unpredictability in the magnitude of the change in common usage. Over time, the understanding has drifted a bit to include, usually, the idea of significant magnitude.