You may have observed an unexpected growth in the database size and transaction log after a cross forest migration. It happened to me when I was doing a cross forest migration project from Exchange 2007 Server to Exchange 2010 server, so I thought of sharing it so that it may help you to plan the migration in a better manner.
The database size and transaction logs growth was unacceptably high due to few users’ outlook client. I observed ~30GB logs generated because of 2 users, and the database is also grown about 25 GB in a time span of 8 hours without any reason.
When I identified the users and clients (used Exchange User Monitor), it is observed that all the client versions are outlook 2010 original version (without any service pack), have mentioned it below.
Microsoft Outlook 2010 OUTLOOK.exe 14.0.4760.1000; I also observed that most of them were in Online mode with exchange.
To quickly work around this issue, all you need to create a new outlook profile for the user.
To plan the migration better, you must install the latest service pack for outlook (at least SP1) as I have not seen this issue with the SP1 version. I also have observed many migration issues with cache mode profile are also fixed with SP1.
How do you Identify the users: