I read the white paper and it was there in black and white
'Data is encrypted in the cipher block chaining (CBC) encryption mode. The encrypted database files that are written to disk are the same size as the unencrypted files because no extra padding is required and the initialization vector (IV) and encrypted DEK are stored within the existing space. Because the log is padded to the next VLF boundary, the log will grow in size. Note that while the database state is marked as Encryption enabled, the actual state of the encryption should be monitored through the DEK state. When the background scan is complete the DEK state is set to Encrypted. At this point, future writes to the log and to disk are protected.'