That specific finding was from what appears to be the 1997 QSM study [0], though the description I am familiar with it from (Mike Cohn’s Succeeding in Agile: Software Development Using Scrum, 2010, describes it as covering projects in 2003-2005 (the author, organization, and number of projects in the analysis, and the one result chart from the study that is in the article on the web [1] match between the book and the 1997 one, though, but the chart showing time to complete by team size isn't shown in the article.)

[0] https://www.qsm.com/blog/2019/4-key-studies-team-size

[1] https://www.qsm.com/articles/team-size-can-be-key-successful...