Thanks everyone for their thoughts and keep em coming.
I think it's interesting how everyone has their own (similar yet unique) definition. Personally, my own definitions are pretty much switched with the general consensus.
For me, a mini-game fits most of the criteria that most associate with a campaign model: it needs to be pretty close to the standard rules, perhaps offer some small chunks of easily-digestible rules (like classes, or a special new mechanic that's not too complicated) and must be short so that I can get it read and ready to play in no time.
A campaign model, on the other hand, is a more sweeping project. It presents a whole new way of playing the game, and introduces new variations in the way the standard rules work. Perhaps it comes with an obvious setting, though at least there is an implied setting in the rules. It requires some careful reading in order to really get the way new rules integrate with the model being presented, and while it doesn't have to be long, it is meaty, since it must present new rules, changes to the standard rules, and setting information (whether an actual setting or suggestions to fit it into ongoing campaigns/genres).
I think it's the inclusion of the word "campaign" that makes me feel it must be a larger work than a "mini" game.
And, actually, Barsoomscore, it was because I saw the Open Call for EN Mini-Games that I got to think of this.
Like I said, your thoughts are very much appreciated and I hope to read more.