That's a great question! During the initial cleanup, the air in the room is cleaned based on the room size and other configuration parameters available in the app (e.g. number of pets and the age of your house).
When you change the room size, Molekule identifies that your cleaning needs have changed. It does an initial clean to meet the new target and ensure that all the air in the room is properly purified. The initial clean will take anywhere from 20 minutes to an hour.