Being overstaffed hurts a team in counterintuitive ways.

DHH (and Fred Brooks) on Twitter's Headcount daringfireball.net

It feels like throwing more engineers at a project ought to make the work go faster. But it doesn't, past a certain point. Fred Brooks published The Mythical Man-Month in 1975, wherein he coined Brooks's Law: "Adding manpower to a late software project makes it later." To my knowledge, there are no known exceptions to this axiom. It is always true.