Understanding storage requirements of a PostgreSQL table - postgresql

Question

I have a PostgreSQL table that stores OHLCV data from an exchange. The table has recently become large and I'm trying to reduce its size by understanding how the different table components contribute to the total size.
This is the table's schema:
+-----------+-----------------------------+-------------------------+
| Column | Type | Modifiers |
|-----------+-----------------------------+-------------------------|
| ts | timestamp without time zone | not null default now() |
| millisecs | bigint | not null |
| open | numeric | not null |
| high | numeric | not null |
| low | numeric | not null |
| close | numeric | not null |
| volume | numeric | not null |
| symbol_id | integer | not null |
+-----------+-----------------------------+-------------------------+
The size of the table without indices and toasts is:
SELECT pg_size_pretty(pg_relation_size('ohlcv'));
+----------------+
| pg_size_pretty |
|----------------|
| 6871 MB |
+----------------+
However, when I get the size of each column and add up the results, I get:
SELECT
pg_size_pretty(
sum(pg_column_size(ts)) +
sum(pg_column_size(millisecs)) +
sum(pg_column_size(open)) +
sum(pg_column_size(high)) +
sum(pg_column_size(low)) +
sum(pg_column_size(close)) +
sum(pg_column_size(volume)) +
sum(pg_column_size(symbol_id))
) FROM ohlcv;
+----------------+
| pg_size_pretty |
|----------------|
| 3769 MB |
+----------------+
This is a fairly large difference in size. I realized NUMERIC is a variable-size type if the precision and scale are not specified, so I figured the difference in sizes is due to column padding. I ran the following query to test my theory:
SELECT
pg_size_pretty((
max(pg_column_size(ts)) +
max(pg_column_size(millisecs)) +
max(pg_column_size(open)) +
max(pg_column_size(high)) +
max(pg_column_size(low)) +
max(pg_column_size(close)) +
max(pg_column_size(volume)) +
max(pg_column_size(symbol_id))
) * count(*)) FROM ohlcv;
+----------------+
| pg_size_pretty |
|----------------|
| 5797 MB |
+----------------+
This query gets the maximum number of bytes for each column and then multiplies that by the number of rows.
The result is closer to the table size but still more than 1 GB smaller.
Finally, there's this query that returns a number somewhere between the two:
SELECT pg_size_pretty(sum(pg_column_size(ohlvc.*))) FROM ohlvc;
+----------------+
| pg_size_pretty |
|----------------|
| 6240 MB |
+----------------+
Can someone help me understand in detail the differences between these sizes? I understand there's overhead per-table, but what accounts for the difference between the results of the last 2 queries?
By the way, I have already vacuumed the table with
VACUUM FULL VERBOSE ohlcv;