Some oppositional word pairs seem needlessly contradictory, even if they can be defended under some tortured argument involving figurative usage. For example, it doesn't make much sense to say
The prolonged cold snap produced a blossoming withering of the plants in the garden.
An author might argue that "a blossoming withering" is an arresting and provocative way to express the progressiveness of the garden's withering as the cold snap continued—to which I might reply, "Okay, sure, sure." But I wouldn't want to read a book full of that kind of writing.
On the other hand, some oppositions—such as "time grows short"—are so thoroughly entrenched in everyday language that we scarcely think of them as involving contradictions at all. And other oppositions work well because they are vivid and not really paradoxical when considered in context:
The trees gradually emerged in the lightening darkness as dawn approached.
Along these lines, "increasing brevity" seems to me to be defensible as a way of saying "for shorter and shorter periods as time went on."
Consider Duration A = 30 seconds and Duration B = 20 seconds. Is Duration A more brief than Duration B or less brief than Duration B—or can we not speak of brevity in relative terms because, somehow, the core idea that brief means "short" doesn't permit the notion of "even shorter" without raising doubts about whether the less short interval can legitimately be called brief at all?
My answers to the questions in the preceding paragraph are (1) Duration A is less brief than Duration B; but nevertheless (2) Duration A doesn't cease to be brief just because we compare it to an even briefer (that is, more brief) time interval. If a new process enables us to reduce the length of time it takes to produce 1 gross of widgets from Duration A to Duration B, and if we for some reason want to talk about the two durations in terms of brevity, then it strikes me as perfectly reasonable to identify Duration B as the "more brief" time interval and to assert that the new process "increases the brevity" of the time interval required for widget production.
You might still wonder why anyone would want to talk about "increasing the brevity" of the process rather than, say, "shortening the duration" of the process, but that objection seems to me to change the question from "Is 'increase the brevity' logically possible?" to "Is 'increase the brevity' the best way to express the underlying idea of shorter and shorter durations?"
Finally, it seems to me that oppositions that intensify or flow in the natural direction of the noun in the oppositional pairing are actually easier to understand than nominally nonoppositional pairings that weaken the noun's natural direction. For example, I have an easier time apprehending the intended sense of "increase the brevity" (which involves intensifying the brevity by making the brief thing briefer) than apprehending the sense of "decrease the brevity" (where decreasing seems to be a natural companion for brevity until you realize that "decreasing brevity" is the same as "increasing longevity").