I tried going the small trunk route on one of my bikes and it felt more unstable, even at a fraction of the weight of my fully loaded winter commuter's panniers. I like having large inexpensive panniers, since I can still make them light as I want, but pack a huge load when I need to at a lower center of gravity. I can also distribute the load weight from side to side for stability, not much of a viable option with a rear trunk. I do carry a smaller backpack on summer commuter, but I may rethink that next summer, it would keep me cooler body temperature wise, and even a mildly loaded pack is hard on the spine. It'll be better to let the bike take the abuse since it is easier to fix or replace than me.