sum of all database files (datafiles) is the size of the database. it is not the ACTUAL SIZE OF THE DATBASE USED.
the following may help you( it is a part of my reporting script..so bear with the formating).
C:>cat sum.sql
COMPUTE SUM OF Growth_in_meg ON report;
COLUMN  growth_in_meg format 999,999,999
SELECT  to_char(creation_time, 'Month RRRR') Month_year,
        TRUNC(sum(bytes)/1024/1024) Growth_in_Meg
FROM    sys.v_$datafile
GROUP  BY  to_char(creation_time, 'Month RRRR');
clear breaks;
clear computes;
set head off;
SELECT 'Size of the database    : '|| TO_CHAR(Round(sum(bytes)/1048576, 2), '999,999.00') || ' MB'  from dba_data_files;
SELECT 'Free space              : '|| TO_CHAR(Round(sum(bytes)/1048576, 2), '999,999.00') || ' MB'  from dba_free_space;
SELECT 'Used space              : '|| TO_CHAR(Round(sum(bytes)/1048576, 2), '999,999.00') || ' MB'  from dba_segments;
set head on;
C:>sqlplus mag/mag

SQL*Plus: Release 8.1.6.0.0 - Production on Fri Feb 7 08:10:24 2003

(c) Copyright 1999 Oracle Corporation.  All rights reserved.

Connected to:
Oracle8i Enterprise Edition Release 8.1.6.0.0 - Production
With the Partitioning option
JServer Release 8.1.6.0.0 - Production

mag@itloaner1_local > @sum.sql

MONTH_YEAR     GROWTH_IN_MEG
-------------- -------------
August    2002           849
February  2003             1
January   2003             1
October   2002           150

Size of the database    :    1,001.75 MB

Free space              :      857.07 MB

Used space              :      144.55 MB

mag@itloaner1_local >