You can set your Sent item to NOT be saved by automatically.

If you prefer to Save Copies of your Sent items, you need to periodically delete them from your Sent Items folder. Otherwise, they will accumulate indefinitely.

Select items to be deleted. You can select multiple items by holding down the CTRL key ( non-contiguous items) or the Shift key ( continguous items)
Press the <Delete> key on your keyboard or choose Edit/Delete from the Menu bar.
Items deleted from the Sent Items folder will go into the Delete items folder and will not save any space until they are permanently deleted from the Deleted items folder.