Why is a specialized group of small cells better than one large cell with the same volume

When there are more components to a system, like having more cells, then there is a greater potential for more complex behaviors. With more complexity, there can be some advantages for survival. The interesting thing is that we still have here on earth, very simple organisms that are composed of one or just a small number of cells, and we also have organisms that are composed of a large number of 'small' cells. The jury is still not out on which are more successful. As an example, if we complex humans with a large number of small cells launch a nuclear attack against each other, we would all die, but the cockroaches would probably survive because, in part, they are made of a smaller number of cells. Which anatomy has a better advantage? It's hard to say.