I thought I was doing overkill with two small panniers: Change of clothes, Lunch, spare tubes etc. I still had room to spare, but not much. Then I read about people bringing clothes, a lunchbox, multi tools, spare chains, spare tires, etc etc.

I think that a large portion of the commute is planning. My lights are USB recharge, so I left spare cables at the office. I used to be able to leave shoes at the office, so I had one less thing to carry. Some weeks I would drive in on Monday and bring lunch for the week and a few changes of clothes, then ride in with my roadie, no bags. It was nice. I had a nice setup with my bike in the office, so I never needed a lock, so it's a variable for everyone.