In case you need assistance in merging all data, you could do it this way:

    DO $DO$
    DECLARE
      _tab text;
      _r record;
    BEGIN
      FOR _r IN
        -- refine the following query according to your particular needs, if the following doesn't work well
        SELECT a.attrelid::regclass::text AS table_name
        FROM pg_attribute a
        JOIN pg_class c ON (c.oid = a.attrelid)
        WHERE a.attrelid IN (SELECT attrelid FROM pg_attribute WHERE attname ILIKE 'geom') AND -- any table with a "geom" column name
              c.relnamespace > 11 AND c.relkind = 'r'
        GROUP BY a.attrelid
        HAVING COUNT(*) = 3 -- only tables with three columns
        ORDER BY 1
       LOOP
         IF _tab IS NULL THEN
           _tab := _r.table_name;
           EXECUTE 'CREATE TEMP TABLE merged_multi_polygons (tablename text, LIKE '||_tab||');';
         END IF;
         EXECUTE 'INSERT INTO merged_multi_polygons SELECT '||quote_literal(_r.table_name)||', * FROM '||_r.table_name
                 -- ||' WHERE size < 20' -- UNCOMMENT THIS LINE TO PRODUCE THE TABLE ONLY FOR SMALL-SIZED ENTRIES
                 || ' LIMIT 1';
       END LOOP;
    END;
    $DO$;
    
    SELECT * FROM merged_multi_polygons WHERE size < 20;