I do this a lot for printing out (research) articles, and the answer is that the text gets shrunk much more, and the margins increased to account for the difference. (This actually turns out to be useful in practice, since there's much more whitespace to take notes in.)
You are misunderstanding him. He's not saying that you can't shrink A4 down to A3. He's saying that aesthetically you can't do this without (of course) changing the font size. But if you want to retain readability, you may need to keep the font size larger, thus requiring a complete reformatting of the text. That is, you're talking about math and he's talking about the realities in dealing with humans as readers.
