table space keeps growing.

My database keeps growing, although the size of the data remains almost
constant. The database is currently using 140GB, although I estimated
the actual size to be around 40-50GB. Vacuum's are run twice a day, as
the superuser, after the large updates and deletes complete. I
expected the size to level off but it keeps growing, what am I doing
wrong? For instance one of my tables started with 546,050 pages and
has grown to 2,290,469 pages although the number of rows are roughly the
same. Does vacuum free sparse space on a page or just the free space at
the end? Also, I'm just running vacuum verbose analyze and not running
vacuum full. Vacuum full does reclaim the space, but it also keeps the
table locked most of the day.

-michael