The Mythical Man-Month posits that adding manpower to a late software project makes it later, because communication and coordination overhead grow faster than productivity (Wikipedia). The book is from the 70s, but it's been proven out time[1] and time[2] again[3] over the years. I've definitely witnessed it myself.
We're now moving into an era where a team of 5 engineers can easily outperform a team of 50 (and beyond).
And this isn't just these 5 engineers using AI tools, whilst the bigger team doesn't. It comes back to the original hypothesis of the Mythical Man-Month. AI dramatically multiplies individual output, so a small team can match the raw productivity of a much larger one—without the coordination tax. Fewer people means fewer communication channels, less time lost to alignment, and more time spent actually building.