Right, RAM an order of magnitude faster than disk, so calculations will be performed very quickly. Big data usually implies clusters of servers because the data won't fit on one server (even on the disk).
Big Data usually implies 'big dollars', not necessarily a large amount of data. Simply use some in-efficient algorithm and a datastore with sufficient overhead and you're in Big Data territory.
Re-do the same thing using an optimal algorithm operating on a compact datastructure and you make it look easy, fast and cheap. Of course you're not going to make nearly as much money.
