- causing vast numbers of people to move across state boundaries in migrations as large as any since the great migrations of the thirteenth and fourteenth centuries
- Investors face a geopolitical situation as dangerous as any since WW2
- crisis as deep and as dire as any since the Great Depression
and so on.
Read literally, I would take it to mean that N is as X as any number of similar events that happened since Y, which will turn out to mean that N is perfectly ordinary. But of course the phrase means something completely opposite and is supposed to say that N is extraordinary; that the world has not seen any Ns as X as the one that happened back in Y.
I cannot understand how the phrase came to mean what it does. Is it the same process as with "I could care less" (where could sneaked in instead of couldn't)? Or is there some other explanation of this phrase's curious wording?