In reply to neuromancer:
I had a 10L (Montane Via Jaws 10, old) which was fine but I couldn't have managed with anything much smaller. You now need a non-collapsible cup; for me that needs a mesh pocket to itself to have it accessible, not dangling around, and not cacking up your other kit. I was on the 100. Not many people had smaller vests, but then I was firmly at the back so missed the elite runners.
Lots of people in the 50 had generous 20L+ rucksacks full of treats and clothes. At first I eyed these with derision, but actually it makes sense if you're mostly walking - it's all about being confident. Lots of people also had (race vest + bumbag); for me this seems sweaty and faffy and heavy, but it clearly works for some people.