While breasts are almost always a little asymmetrical and have different milk-holding capacities, such a huge difference might have to do with your feeding habits. Mom and/or baby might have developed a preference for one breast, feeding more (or more often) on that side. This can lead to lopsided milk production. You might also become lopsided if baby refuses one breast for a while (sometimes due to an ear infection).
If your uneven breast size is bugging you, you can try nursing more often on the smaller side to help even things out. Remember that milk production is a supply-and-demand deal. The more milk you remove from a breast (by nursing baby or pumping), the more milk it will make. Likewise, nursing less on the larger breast can help tame your supply on that side. (Don't ignore that breast altogether — just nurse a little more often on the smaller side.)