As the pump rotates, the 3 contributing plungers deliver fuel to the rail. The delivery is split into 3 evenly spaced pulses, 120* of crankshaft rotation in seperation.  Although these pulses overlap each other, they still have their pressure peaks. With a small factory single line the pulse is emitted in a longer frequency duration of higher magnitude, the larger the inside diameter of the line the slower and lower the frequency. As the injectors fire, they emit an opposing frequency into the rail.

An anology for this can be envisioned as a group of runners on the starting line, with a smaller line it would be like runners taking off very quickly, the first one would go 20 feet then the second one would take off and so on. With a larger line it would be the same scenario with the runners leaving the line more slowly and at closer intervals. Since we are only dealing with volume at a given pressure, the larger line has some ability to counteract the contribution of the injectors shockwave pulses emitted into the rail. Imagine a line of people running through a door, and very frequently the door slams shut. What happens to the line of people? If that isnt enough, the line that is forming for the door, is just like the runners leaving the starting line, running very fast into the line of people. With a single larger line, the haste of delivery is virtually eliminated, although it does not eliminate the injectors contribution to the frequencies generated, it does not make it worse, and can actually absorb some of the frequency like a shock absorber.