XML-binary Optimized Packaging (XOP) is a mechanism defined for the serialization of XML Information Sets (infosets) that contain binary data, as well as deserialization back into the XML Information Set.
This size increase results in extra resources needed to transmit or store the data.
A packaging mechanism defines how the XOP Infoset and the binary chunks are represented.
Those other MIME parts can be serialized as binary data, avoiding the need to base64 encode them if they were left inside the XML Infoset.
The MIME packaging mechanism is the most widely used, since XOP is usually used to represent SOAP messages with MTOM.