They do not. Each joining spouse and each unmarried child under 21 must be considered in the initial application and carries its own nonrefundable US$15,000 fee and its own US$1 million gift, including where the principal comes through a corporate sponsor. Household totals therefore scale with the number of people, not with a single headline figure.
Rebuild the household total from scratch
Rather than adjusting an old estimate, write the list of people and multiply. Confirm each person's status against the definitions used in the official instructions, a spouse and an unmarried child under 21, and ask counsel about anyone whose position is unclear, such as a child close to that age. Then check the total against the current published figures on the day you plan to pay, not against the number that prompted the plan.
Build the list of people before the arithmetic rather than after it, because the composition is what changes the total and it is the part most often left provisional. Ask counsel about anyone whose position is unclear: a child approaching the age limit, a stepchild, an adult child, or a partner not legally married. Because joining family members must be considered in the initial application, adding someone later is not a straightforward top-up, and the decision about who is included is therefore made once, early, with the full figure in view.