I'd like to know by whom that's been said.

What's true is that a tire that is claimed to measure 25mm does not always measure 25mm. Sheldon has a good article on this, too. So though you'd be hard pressed to find a 25mm tire with a markedly smaller contact patch than a 23mm, you might find one such pair that measures about the same width-wise, and that both give a contact patch that's more or less identical.

The prevailing opinion on this thread is good advice, I think. For city riding, you'd do well with a higher-volume tire pumped up to a lower pressure. As for me, I'm on Continental Town & Country's, which measure two inches across, or about 50mm, to put things in perspective.