But they must be wicker and you must wear a scarf and no helmet, for max style points.

+1 to Topeak MTX w/ expandable drop-down small pseudo-panniers. Easy on/off the bike, handles the usual small load well, surges acceptably to the medium load days. That is my preferred option for most days.

+1 to having other options for those unusual days. A setup that works 90% of the time leaves you hanging 10% of the time. Messenger, pannier, move the trunk bag and lash the computer bag to the rack, etc.

I am highly suspicious of the false packing discipline that having a too small bag enforces. Unless you are an unbearable pack rat, you are probably going to cut bone. And if you are an unbearable pack rat, maybe you find bliss in having all that crap with you. Only go light if you are comfortable with a c'est la vie attitude to being a little too hot / cold for the temperature swing, getting a little wet, wearing brown shoes with a grey suit, etc. As we used to say in another context -- travel light & freeze at night. Works for some.