Literary analysis: Once again Isaiah presents paired images. The first is the contrast between the sands of the sea and a "remnant." The second is the contrast between the "consumption decreed" and "overflow with righteousness." Each of the two members of each paired set fit into a conceptual unit.
In the first, the contrast is between numbers, with a very large number contrasted to a "remnant" which is necessarily not only smaller, but by implication, much smaller. In the second set, the consumption or destruction is contrasted with righteousness. One again however, we have not a simple contrast, but one of a destruction to a righteousness that is "overflowing."
By reading the paired images both separately and together, the sense of the passage is that just as the large number will have only a small remnant. so will this small (in the Lord's perspective) destruction have an abundant compensation in righteousness.