The files owned by a Google Service Account were trashed and permanently deleted from the trash by my normal user account; Service Account storage space was not reclaimed as expected. Existing files owned by the Service Account adds up to 4.56 GB (4557015060 bytes), the about Google Drive API response says the Service Account usage is 16.08 GB (16084076368 bytes). The difference is 11.52 GB — what I should have gotten back.