This certainly used to be true, back when ram was expensive and drives were slow. But progress on both of those fronts has massively outpaced the increase in typical structured data, which is what you would store in a db. It's rich/unstructured data that has increased hugely in most usecases. But I would argue that it's not as cut and dry nowadays, particularly with the amount of serverside processing that is being done in other non-db areas.