I found, at this link -http://docs.oracle.com/cd/E15523_01/web ... 4/mtom.htm
This seems to suggest that the size should be reduced when using MTOM?Binary content, such as an image in JPEG format, can be passed between the client and the Web service. In order to be passed, the binary content is typically inserted into an XML document as an xsd:base64Binary string. Transmitting the binary content in this format greatly increases the size of the message sent over the wire and is expensive in terms of the required processing space and time.
Using MTOM, binary content can be sent as a MIME attachment, which reduces the transmission size on the wire.
I had read it is approx. 30%