Hello, after each website I delete the history but although the storage volume becomes bigger

Hello. I installed Firefox Focus today new on my IPhone. The used memory for “documents and data” was 5,8 MB. But now after only one website and deleting the entire history, the used memory for “documents and data” according to settings/general/iphone-storage is already 8,5 MB. And I deleted the entire history. How can this be? What does Firefox Focus saves without mu knowledge? Siri is not active and the fast search also not! What also is Firefox Focus doing??? Thank you for help