Why is tuple_percent so low?

I am trying to refactor a table on disk so it consumes less space:

Original is:

create table post_timings(
   topic_id int not null,
   post_number int not null,
   user_id int not null,
   msecs int not null
)


Target is:

create table post_timings(
  post_id int not null,
  user_id int not null,
  dsecs smallint not null
)


Before I have:

select * from pgstattuple('post_timings2');

 table_len  | tuple_count | tuple_len  | tuple_percent |
dead_tuple_count | dead_tuple_len | dead_tuple_percent | free_space |
free_percent

------------+-------------+------------+---------------+------------------+----------------+--------------------+------------+--------------
 5146427392 |   116221695 | 4648867800 |         90.33 |
 0 |              0 |                  0 |   15082484 |         0.29


After I have:

 table_len  | tuple_count | tuple_len  | tuple_percent |
dead_tuple_count | dead_tuple_len | dead_tuple_percent | free_space |
free_percent

------------+-------------+------------+---------------+------------------+----------------+--------------------+------------+--------------
 5142036480 |   116122544 | 3948166496 |         76.78 |
 0 |              0 |                  0 |   15069224 |         0.29


What I find striking is that the table size on disk remains almost
unchanged despite tuples taking 6 less bytes per tuple.

All the "missing space" is in overhead that is missing from
pgstattuple, in particular tuple percent moves from 90 to 76.7

I was wondering:

1. Where is all my missing space, is this in page alignment stuff and
per-page overhead?

2. Is there any other schemes I can look at for storing this data to
have a more efficient yet easily queryable / updateable table.

Keep in mind these tables get huge and in many of our cases will span
10-20GB just to store this information.

Sam