I didn't intend to say that you can't get a good estimate of the size of the data/transactions/etc for any projects. Just that, all too often, the estimates will be low.

I've encountered situations where it was categorically stated during design that something was out-of-scope and then, a few months after go-live, management decided to bring it into scope. Also, I've had cases where a detailed analysis and projection of volmes based on existing and anticipated business is done and then, after implementation, unexpected new business turns up which throws out all the projections.

Basic rule seems to be to assume that all volumes are significantly underestimated!