size3

Nice — that explains it. Your previous query listed every table (including partition children), so the parent rows looked wrong.
Below is a query that returns only top-level (parent) tables and aggregates the sizes of their partitions (plus the parent itself) — result shown in GB (rounded to 2 decimals):

WITH RECURSIVE descendants AS (
  -- start from top-level tables (not a partition child)
  SELECT c.oid AS parent_oid, c.oid AS child_oid
  FROM pg_class c
  JOIN pg_namespace n ON n.oid = c.relnamespace
  WHERE c.relkind IN ('r','p')  -- ordinary + partitioned tables
    AND n.nspname NOT IN ('pg_catalog','information_schema')
    AND NOT EXISTS (SELECT 1 FROM pg_inherits i WHERE i.inhrelid = c.oid)

  UNION ALL

  -- recurse to find all descendant partitions
  SELECT d.parent_oid, c.oid AS child_oid
  FROM descendants d
  JOIN pg_inherits i ON i.inhparent = d.child_oid
  JOIN pg_class c ON c.oid = i.inhrelid
)
SELECT
  ns.nspname   AS schema_name,
  p.relname    AS table_name,
  ROUND(SUM(pg_total_relation_size(d.child_oid)) / 1024.0 / 1024.0 / 1024.0, 2) AS size_gb
FROM descendants d
JOIN pg_class p       ON p.oid = d.parent_oid
JOIN pg_namespace ns  ON ns.oid = p.relnamespace
GROUP BY ns.nspname, p.relname
ORDER BY SUM(pg_total_relation_size(d.child_oid)) DESC;

Quick notes:

• This returns only top-level tables (regular tables and partitioned parents). Partition children (individual partitions) are not listed separately.
• For non-partitioned tables the size_gb is just that table's size.
• The query uses a recursive walk of pg_inherits so it works for multi-level partition trees.
• If you want to restrict to a single schema, add AND ns.nspname = 'your_schema' in the first CTE WHERE clause.
• Running pg_total_relation_size() for many objects can be a bit heavy — expect the query to take some time on huge databases.

If you still see unexpected values, run this quick diagnostic (replace schema.parent_table):

SELECT
  pg_size_pretty(pg_total_relation_size('schema.parent_table'::regclass)) AS parent_size,
  pg_size_pretty(SUM(pg_total_relation_size(c.oid))) AS sum_of_partitions
FROM pg_inherits i
JOIN pg_class c ON c.oid = i.inhrelid
WHERE i.inhparent = 'schema.parent_table'::regclass;

Send me the output if the numbers still look off and I’ll help debug further.