As a lay person I've wondered this for a while. Obviously heavy machinery that rolls on dirt and mud needs large tire/wheel diameters and widths to spread their weight out to an acceptable ground pressure, eventually having to switch over to cat tracks when larger wheels start to give negative returns. But what about trains? They roll across the hardest, smoothest surfaces that anyone could reasonably wish for, but their wheels are still proportionally quite large. Why? Why are bicycle wheels so huge? At least for road bikes; for mountain bikes it makes more sense.