A flatter-than-planned cookie can point to the dough, the pan, or the oven. Start with one test tray.
Cookies can spread too much when their fat melts before the dough sets. Warm butter, a measuring error, a hot pan, and an oven running cool are common suspects.
We portion our larger cookies with a scoop and weigh them, but our baking practice is not a controlled comparison of all spreading causes. Change one variable and test a single cookie before adjusting a batch.
King Arthur Baking documents how temperature, flour-to-fat balance, and pan lining affect cookie spread.