I P Go to User Preferences and set 'Default Mail setting' to 'Send Only'. This will prevent every mail being saved that you send and keeps the size of your mail file in minimum. You can separately save copy of each message when sending. For Your information Mail boxes are automatically compacted once a week. Compaction cleans the space reserved by deleted entries or other such unrequired data.
Compaction takes place during weekend.
While compacted, your are not able to open your mail box. This may take couple of mininutes, so try again after a short period. You will see the current size of your mail box from the bottom of the Inbox view. So called shared mail feature is enabled for the mail server. This means that even when you receive mail, the size of your mail box does not always increase in respect to the size of the message your have received.