I know how mandrel bends work (not really, but w/e), I'm more concerned with how much smaller the tube is when it's bent/curved than when it's straight pipe. Yes, the ID may still be 2" at all points in a mandrel bent pipe vs a normal straight 2" pipe. But in a curved pipe that's 2" ID, it'll 'act' like say a 1.75" straight pipe.

Kinda like Bisi's Headers for the Honda D-series. It's a bunch of larger bends that 'act' like a smaller diameter straight pipe. Also somewhat like the picture below, the bends are a marginally larger diameter than the straight parts.

I'm wondering how much 'smaller' the bent pipe is compared to a straight pipe of the same size.