For a short trip, I used to throw a few things into a bag and leave. But now I always seem to pack a lot of stuff. One reason is that I'm older; I need more stuff, and I remember times I missed bringng x or y, so I'll pack those.
But the real villain is my wonderful packing list. I go through my spreadsheet listing everything I might need for the most complex of trips and ... oh, I might as well take that, this, the other, those, these and etc. And so my bigger suitcase fills up, just in case.