Well,
This is a database issue. I have no knowledge of Informix, but with DB2 I would look for the following parameters:
1. Numbers records within a table
2. Check numbers of pages occupied by table (with DB2 standard page size is 4kb)
Alternatively:
Check size of tablespace that holds the tables.
Run statistics on all tables and read the size of them (pages,Mb's)
Usually the datatables will form the bulk of space occupied , with some space for system tables.
Now, for ORACLE I would tell a different story...
Informix forum will tell you how things are dealt with for that specific RDBMS..
Ties Blom