Reclaiming Unused Space In A Table

I'm trying to figure out how to reclaim unused space in a huge table (tbl) after setting one of the text columns to be the empty string. Other tables have foreign key columns associated with tbl.

After doing something like this,

update tbl set col = ''

the table doesn't automatically reclaim that space. If I do a direct insert into a new table like this,

insert into tbl2 (cola, colb, colc) select * from tbl

the new table is smaller as expected (about half the size in my case). Any ideas?

Rob