Large roofs equal large water flows. This is where a full flow system really comes into play. You get rapid drainage without the use of large pipes. Local dimensioned rain is what controls calculations.
A UV-system is different from a conventional gravity system in the way that the dimensioned rain does not let air into the management system. Since the air takes up about 60-70% of the area in a gravity trunk, you can reduce the pipe dimensions by about half with a UV-system. The pipes can be laid horizontally without compromising the function. In some cases the tubes may also be installed in the opposite direction. By increasing the fall height, the capacity is also increased. You could call it a siphon.