I always try to use the smallest bag I can get away with. This is the kind of where you have room for what you need, and not much more.
My previous bag was perfect for what I needed then. But it is too small because I have to bring my computer to and from work at my current job, something I didn’t need to at my previous.
The one I am using is way too big, and I haven’t found the new “perfect” yet.
My problem with a large bag is that it leaves room for carrying a lot of junk. The result is two new problems, first it becomes full, and then you can’t find anything, because it is full of junk.
The result is that I have to empty it once a week, put back what I need and figure out what to do with the crap.
The thing I like about using a bag where you don’t have much more room than what you need is that you can’t fill it up with crap, and you need to consider every singe thing you want to add. It comes down to how important that new thing is, and figure out what to do. You could chose to not add it, or you could remove something else or get a bigger bag. It all comes down to how important it is.
It always feels like I am doing better choices when I really have to consider everything up against each other. This is one of the ways I try to force myself to do just that.