No I don't think so. From my understanding replays store coordinates of everything that's going on around you. So for example it saves 30 lines of coordinates per second of only you driving (with ghosttown). But if you turn ghosttown off it will save another 30 lines of some ped walking around, and then another ped and then some random cars. And because replays are locked at some size (700 or 800 something kb, I don't remember the exact number), they will be shorter because they will save too much of random infos (mentioned before, cars and peds). It's way more complicated than that but it should give you an idea why ghosttown helps.