For me, "big" data is increasing the linkage between your data. It's not simply more data, but much richer, less formal data relationships. It's taking your sales data and linking it to your website clicks, linking that to the weather (or whatever). Or you take something traditionally static and add a temporal dimension.

This kind of deep linking you can't measure with straight megabytes. A few gig doesn't seem that large, but if it's a complex graph with a complex hypothesis - then, sure - that's big.