I don't have a massive math brain but I have a hard time considering those funny equations, I'm not seeing complicated integrals anywhere, which I would expect considering the game simulation happens over time, and in 3D.
It feels like an easy way to either make it seem like the difference is small because the resulting number is small, or the difference is huge because number is huge.
Even the difference in volume is not properly evocative if you forget/don't know how volumes scale.
If you really cared to have a good measurement, I think Pheaa's got the right idea about it: "I would love for someone to create a plugin that checks if an iron bomber pipe that hit would have hit if it had the stock hitbox."
However, you could only easily find which default-sized pipes would have collided if they were IB-sized, so either with the resize feature already enabled, or by checking non-IB pipes (which may skew your data, as people mention that IB pipes are harder to spot mid-air, so harder to dodge). This is potentially a resource hog.
You could find the opposite if you managed to simulate the game as if IB-sized pipes were actually default-sized, and check whether they would still collide over their lifespan. This is pretty much only feasible as post-processing analysis of demos, and a huge amount of work.
At this point though, it feels like we're doing a lot to find whether a bugfix should be applied or not, is this the right criteria for inclusion?