Custom Data Type size - too big overhead?

Hi,

we are trying to minimize data storage size as possible. We'd like to
replace BOX and POINT datatypes with our own. We don't need double precision
- 4 bytes integer would be totally fine.
I tried following experiment in which custom data type vPointInt of 4
integers takes 28 bytes. Why is that?

Create Type vPointInt AS(
a integer,
b integer
)

CREATE TABLE rt(
id int,
pt vPointInt
);

insert into rt values(1, (4,4) );
select *, (pg_dump(pt)).* from rt limit 15;

1;"(4,4)";"vpointint";30712;28;"000,000,000,002,000,000,000,023,000,000,000,004,000,000,000,004,000,000,000,023,000,000,000,004,000,000,000,004";"\000\000\000\002\000\000\000\027\000\000\000\004\000\000\000\004\000\000\000\027\000\000\000\004\000\000\000\004"

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