What size hydration vest do I need for marathons versus ultras?

Across the reviewers we read, the rough consensus is that a 4-6L minimalist vest covers road marathons and training runs up to roughly a half-day, while 12L+ packs like the Salomon Adv Skin 12 are favored for ultras where you must carry mandatory kit, layers and more fuel. Specialist trail communities repeatedly note that a 5L pack is 'sufficient in most cases' and that runners tend to over-buy on capacity.