I often just take my small Fuji Xt-20 camera when I'm out and about. It's small and so light that there is no reason not to bring it. The small Agile Messenger is a perfect fit and still allows me to bring two lenses. If you have a larger sized mirrorless camera then the medium Agile Messenger is a better fit.
There are times when I want to bring my DSLR - photo trips, work, or if it's low light. For these trips, I’ll bring either my medium or large Agile Messenger. Both carry a surprising amount in a compact space. If I am carrying my 70-200 f2.8 I will definitely take the large.
All of the bags are strong and have weather-proof zips, yet they add hardly any weight to what you are carrying.