I created a smart list for a new email program and it contained over 8000 contacts. After being sent the list seems to have diminished to about 1400 contacts.
Where are you seeing these numbers? Screenshots would be helpful - as well as the filters used in your smart list.
Assuming you have not modified the smart list after the send, my guess would be that you have a filter in the smart list that has now disqualified all but 1,400 of those original 8,000. Perhaps a certain value was changed after the email was sent.