Is the 403 cargo capacity listed separate from the 40 passengers mentioned in details?
Cloud Imperium
This cargo is including personal cargo storage (overhead bins) as well as main storage (below decks.) It does not include the space used by the actual 'people' on the middeck. This ship is massive and has tons of space; if the seats and cabins and other equipment were removed, the cargo space would increase beyond the stated 403.
starfarer.space · Q&A: Genesis Starliner - Part I