database size estimates

Hi,

I am trying to estimate the size of a table composed of 51754000 rows.
Each row has 31 attributes: 16 x bit(24) and 15 x bit(8)

So, the payload should be:
51754000 x ( 16 x 24 + 15 x 24 ) bits = 3115 MB

Now, from what I understand from postgresql manual is that the overhead
is composed of
32 bytes per row + 20 bytes per page.
This leads me to approx. 1700 MB overhead.
Therefore a total table size of 4900 MB.

However, when I load the table, the table reaches the size of 21500 MB,
i.e., 400% of my estimate.
The table has no toast table, no index.
So I am wondering if someone could give me a better estimate.

Cheers,

Francois