If 1 cook can make a cauldron of soup in 3 hours, how long would it take for 10 cooks?
If your assignment is to dig ditches, adding manpower to it speeds you up. This is what we were taught in school. However, adding 10 cooks to a kitchen will not make your cooking 10 times faster. In fact, it will slow your cooking down. You are better off having far fewer.
Most real world projects do not scale linearly with manpower. Yet, when we start a large and urgent project, it is tempting to add manpower from the very first day to speed it up. And when the project is invariably delayed, we add more people to it. This only aggravates the problem.
More real world projects resemble soup making more than ditch digging. When a large soup making project is late, adding more manpower only makes it later.