If you want to create new content database for web application using New-SPContentDatabase cmdlet you should be aware of one side effect related with this action: new content database will be created with Full recovery mode by default even if your basic content database has Simple recovery mode:
With Full recovery mode transaction log will grow quite fast until you will do full backup. It may be significant consideration for large content databases. So before to start using it you may want to change recovery mode of additional content database to Simple. If you moved existing site collection there using Move-SPSite – transaction log already may be quite big. So after changing recovery mode you may also need to shrink log file.