The displacement has a lot to do with the additional power. Why do you think Ford added more cubic inches. If you want to stay NA, power has to be created from something because you can't draw air into the motor any greater than atmospheric pressure. Therefore to make more power, you can only redo the heads so they flow better and/or increase the displacement. Air is the key. The more air that moves in and out of the motor, the more power that can be made. People realized this concept along time ago and hence the turbo, supercharger or nitrous. The realized that is you pressurize air (or increase the density), you can then feed more into the motor. This additional air plus a larger amount of fuel and a spark = more power. Just my take on this...
I'm not saying that the extra displacement didn't help, but I also don't think it was the biggest factor.
There were a lot of guys on here that had stroker / bored out blocks to over 5.0L with huge lift billet cams/heavily ported heads and they were barely making what a bolt-on Coyote motor makes.