Big number of "unused" pages as reported by VACUUM

Hello!

Some time ago I've got troubles with performance of my PG.
After investigation I had found that the most probable reason was the big
number of "unused" pages. Below follows what VACUUM reported:

=======================
vacuum verbose goods;
NOTICE: --Relation goods--
NOTICE: Pages 15068: Changed 0, Empty 0; Tup 16157: Vac 0, Keep 0, UnUsed 465938.
=======================
select count(*) from goods;
count
-------
16157
=======================

The same schema with the almost identical number of rows gives completely
different result on another table:
=======================
vacuum verbose goods;
NOTICE: --Relation goods--
NOTICE: Pages 912: Changed 0, Empty 0; Tup 11209: Vac 0, Keep 0, UnUsed
19778.
=======================
select count(*) from goods;
count
-------
11209
=======================

Two questions:

1) Where to seek the real source of the enormous big number of unused
pages?

2) How to shrink the table (i.e. how can I get rid those unused pages)?

PG: was 7.2.1, now 7.2.2.

--
WBR, Yury Bokhoncovich, Senior System Administrator, NOC of F1 Group.
Phone: +7 (3832) 106228, ext.140, E-mail: byg(at)center-f1(dot)ru(dot)
Unix is like a wigwam -- no Gates, no Windows, and an Apache inside.