Doubt with physical storage being used by postgres when storing LOBs

Hi all

I am developing an application which uses postgres 9.2 to store binaries as
oid objects.

CREATE TABLE content (contentname text, contentoid oid);

I am making some tests to evaluate how much HD space I will need to
allocate these objects.

To measure the space used by postgres I have used two different tools, both
with the same results

1.- Checking physical HD space by making a "sudo du -sb
/opt/PostgreSQL/9.2/data/base/" before and after inserting the data

2.- Asking directly postgres about the tables size estimation "select
pg_size_pretty(pg_relation_size('pg_largeobject'))"

I have tested with different binaries and I am getting different results,
for example when I put the content of a zipped file of 17MB size, the
increment of the disk space is of 24MB. The reason for this increment seems
to be an index created on the table "pg_largeobject". The index is
"pg_largeobject_loid_pn_index"

In other hand when I put let's say many zeroes (same 17Mb) the increase of
HD usage is much smaller.

I think it could be caused because TOAST compresses the content stored, se
he can compress the zeroes but not the previously compressed zip content.

My question is: Is this increase of ~40% normal? Has someone else
experienced this?

Thanks in advance for your help!

Víctor Cosqui