PST file size unchanged for a few days

I am using Win XP Pro SP2 and Office 2003. My pst file stopped increasing in size since last week. The file size is only approx. 5.6GB.

I have a practice to back up the pst file everyweek and it used to change its file size each time I closed the outlook. I have tried running compact but the size remain unchanged (the size was exact match to my last backup file down to the last byte). It stays the same although I have been working with my email everyday.

Please can anyone suggest what might cause this?

Thanks