Actual delivered Message size varies between two different exchange forests.

Hi All ,

In my current environment we have one exchange forest with exchange 2013 and another exchange forest with exchange 2010.

Lets say a mailbox user from exchange 2013 forest is sending an mail along with attachment of (1.7 MB) to one of the mailbox user in same exchange 2013 forest and also to one of the mailbox user in exchange 2010 forest .

So on such case the size of the delivered message to the both the mailbox users in the different forest varies.We have found that with the help of message tracking logs in both the forest.

 I mean the mailbox user in the same forest (i.e exchange 2013) receives an message with a size around 1.7 MB and the mailbox user in the other exchange forest (i.e exchange 2010) receives an message with the a size around 2.25 MB .

Questions : 

We would like to know that how the size of the delivered message varies between the mailbox users in the different exchange forest ?

Is this is the default mail delivery behavior in exchange between two exchange forest ?

Note : On my end i thought that the message from the exchange 2013 forest to exchange 2010 forest was processed two times by the categorizer (one time in exchange 2013 and one time in exchange 2010) because of that size is varied .But i am not sure on my end .

All of your suggestions are much appreciated .

Thanks & Regards S.Nithyanandham