Huge mailboxes after import in Mavericks

Good evening,

I have a strange issue that I haven't read about before.

I'm importing mailboxes from Tiger to Mavericks. Those mailboxes ranges from a few MB to 25 GB. The problem is that after import their sizes is roughly multiplied by four! The complete Mail folder is over 60 GB, you can therefore understand my problem.

Any advices?

Thanks,

Aurelio

 

Same problem importing them first in Leopard.