Disk space usage

I created a table in 8.1.5 on Linux with three columns;

date, bigint and integer.

Then I populated the table with more than 2 million rows.

I looked at the size of the file that contained the table and divided this
by the number of rows which gave an average of just over 60 bytes per row.

This seems to be quite a large overhead as I would guess that the date field
takes around 4 bytes, the bigint 8 bytes and the integer 4 bytes. I would
have hoped to have had an average size of between 20 and 30 bytes per row.
Is this normal and is there any way of improving this as I'm hoping to use
have around 80 million rows in a table without it taking up too much disk
space and too much memory to cache it?

Regards

Joe