3" Xpipe to 2.5" tail pipes?

Seems like I read the same thing several times. That the farther back in the exhaust system you go, the smaller the pipe can be because of shrinking exhaust gases. This is why you want the muffler all the way at the rear of the car, so the muffler does not have to be responsible for handling as much cfm. But, running 1000 cfm of exhaust gas does not seem that it would have much chance to cool off.
.