This is something I've been thinking about a lot lately. I carry this stuff every single day, and while I use everything in my bag, it does kill my shoulders. I carry my wallet, keys, iphone 6, kindle fire, and clear plastic pouch (a small one) that holds my ipod, earbuds, kindle and phone chargers, another small clear plastic zipped pouch that holds the medication and inhaler that I need every day. I also carry a personal sized Filofax, though that is about to change back to an A5 size, as it is a planner I use for both work and personal stuff. My glasses case and sunglasses case, plus a small tube of lotion and another very small zippered bag that holds chapstick, lipstick, a small mirror, fingernail clippers, extra ponytail holders and a few bandaids, and a pencil case that holds a few pens, a high-lighter, and extra lead for my mechanical pencil. Also, lately I've been putting in a library book to read on my lunch break (I mostly read my Kindle, but I also like to listen to it all day with my earbuds while working and am currently reading several library books).
I have a medium sized classic messenger bag from Timbuk2, that I love, love, love, but it sometimes feels like I am loading up a bag for school. I also have a small classic messenger bag from there, but it can be tight fitting everything in.
Any suggestions? I wish I could make myself downsize, but right now I use the things in my bag every day.