Within Classic Single Email Reports, I've clicked the option to Create Database of Contacts. However, the newly created list does not contain as many contacts as I expect.

Why could the number be different?

The Create Database of Contacts option generates a new database of contacts who are still opted in to the database you originally sent to. For example, if A contact opened the email, and then later opted-out from the database, creating a Database of Contacts would not insert them into the newly created database (since they are in an opted out status).

If your number of contacts in the newly created list is less than expected, you may want to check on what email addresses are missing and then look within the original database to see if those contacts are in an opted out status.