Fatal error TooManyLargeItemsPermanentException has occurred

Hi,

I am doing an Inter Forest Migration and I have an Exchange 2010 environment in the source and an Exchange 2013 environment in the target.

I am using the New-MoveRequest command to migrate the mailbox and the process is working fine but every now and again I am getting mailbox failing with Fatal error TooManyLargeItemsPermanentException has occurred.

These messages that are failing the migration are from 30MB to over 100MB in size.  I have work arounds like 

get-moverequest 'JOE BLOGGS'|set-moverequest –LargeItemLimit 3

resume-moverequest 'JOE BLOGGS' 

My questions are -

1. where are these sizes determined ?  
2. Can they be increased ?

Thanks