Think of a five inch pipe, put two 2.5inch pipes inside that pipe next to each other, there is the 5" diameter. now you have the space above and below the internal pipes.

So you can now put another two 2.5" inch pipes in the 5" and you have 4 2.5" pipes in the 5" pipe.

If I could draw it, it would be easier to see. hope that helps simplyfy it a little.

^^ well said.