DataContract size discrepancy

I am noticing a size discrepancy between the size of the data
serialized via DataContract and the size of the data being set through
IIS.

When I look at the IIS log of the size sent for a specific WCF call it
is showing as around 2.5 MB. When I take the data that is being sent
and manually perform serialization using DataContract then saving the
serialized data to the file system the file size is substantially
less, around 685 KB.

Can anyone explain the excessive, additional size of the data as
reported by IIS?

Thank you.