Papertiger is right on the money!
I commute everywhere so I usually carry a tote or a large shoulder bag for work and whenever I'm going anywhere else than grocery shopping or a date with friends.
I need to fit my agenda, wallet, "what if"-pouch (medication, tissue, bandaids, foldable hairbrush, perfume etc etc), sunnies, makeup bag, snacks, water bottle and also umbrella and/or scarf. Lately I have been rocking a warm rainproof coat to cut back on the extra gear though.
My work takes me around the country every day, so I can't just leave anything behind at the office because I might be 400 km away when I need it. I also carry a backpack for paperwork, money and safety gear so it's a lot of stuff to lug around! I love my job but it's taking its toll on my shoulders and neck, so a mini bag is a nice way to "celebrate" my days off.
If I am indeed going to go out for a short while and won't be buying anything (as in, I don't need to put anything in my bag to keep it from getting lost - I also hate having to carry several bags so prefer to put my shopping in a large handbag) I gravitate towards my tiny bags. My wallet is pretty small and every now and then, I switch into a mini to make my bag even lighter. I love the way a tiny bag is light and I don't have to worry about my bag knocking over anything like I have to with a huge tote.