Raw size

Hi,

i have a table:

create table triples(
att0 varchar(1000),
att1 int4,
att2 varchar(20),
att3 varchar(1000)
)

My table has 990 raws.

The (possibly wrong) way, with wich i compute the size of the table is:
att0: 1000 * 1 Byte + 4 = 1004 Bytes
att2: 20 * 1 Byte + 4 = 24 Bytes
att3: 1000 * 1 Byte + 4 = 1004

2032 Bytes + 40 (for oid) = 2072 Bytes

990 * 2072 = 2,051,280 Bytes

BUT after clustering triples according to an index on att1:

select relname, relpages from pg_class ;
relname | relpages
---------------------------------+----------
triples | 142 (8KB/buffer)

142 * 8 * 1024 = 1,163,264 Bytes

Is there any compression or what?