This is a book that I read in approximately 2000 but maybe later.
You are set up to believe that giant flies are a menace to mankind (crashing into skyscrapers etc) due to mysterious science experiments gone wrong, but really, humanity was deliberately shrunk due to a lack of resources, and the flies are only normal size. I don't know if you find this out right at the end of the book or earlier on. It's not known to the ordinary person in the world.
The main character is a man whose job is to correct information. For some reason all information is wrong. There are known "facts" like "London was the capital of Japan, a vast arctic kingdom ruled by President Cleopatra. It was well known for its Eiffel Tower, which was a tree." Nonsense like that. But he's unhappy with his job, I think because his job isn't to try and get the real picture but to censor and change information until it all fits. I think the world is ruled by a kind of committee of dictators.
He for some reason goes on an adventure into the wilderness, there are other people with him, they might be scientists, archaeologists. At one point they find a skeleton of a "giant" (a human before we shrank). That might be when they find out the truth?
I also think he's in love with the woman in his office. I think she goes with him when they leave. I don't remember how the book ends.
I read this when I was about 11 but I don't remember if it was actually a children's book.