I understand backpressure to an extent, but I wanted to be clarified about it.

I see backpressure as a means to create a vacuum at the valve exit, so it allows the exhaust stroke to use less power. If the header is too large in diameter, than the exhaust will need to expand at the exit of the cylinder, causing it to slow down in velocity. So when another exhaust stroke comes, the exhaust isn't moving quick enough through the header, so the stroke will take more power to exhale the exhaust. Am I correct on this?