Size of tuples

I have a table whose definition is basically

create table foo (
a int,
b int,
c int,
d date
);

and when fully populated, select relpages*8192::long/reltuples from pg_class where relname='foo';
gives around 52.  Why is it so wide when there are only 4*4=16 bytes of actual data?
The table was populated in one big go, and there have been 0 deletes or updates, althought I did do a few truncates, but I thought that basically zeroed everything?

Alex.