https://www.verywellfamily.com/how-much-does-it-cost-to-raise-5-or-more-kids-5075170

> Researchers found that families with one child spend 27% more on the only child. Families with three or more children spend 24% less on each child.

Anecdotally kids from larger families I knew growing up often had older children support parenting duties, such as covering for an hour or two between school and work, or driving siblings to school once they got their license.