There are two other factors, perhaps the main reasons. First, the GTi has a smaller diameter tire (225/40R18 vs. GLi 225/45R18) and therefore revs higher. Second, gearing is different. According to 2019 specs listed at C&D for DSG equipped cars, the GTi has a final drive ratio of 4.77 and a 7th gear ration of 0.64. The GLi has a final drive ratio of 4.17 and the same 7th gear ratio of 0.64.
So in addition to the tire size, the gear ratio also means the engine is revving quite a bit higher. The GTi is a bit lighter in DSG guise, 3,128 lbs. vs. 3,217 lbs., but this doesn't equate to better fuel economy because of the big RPM differences. Obviously the two were designed for a bit different market segments.