Does dba_segments show accurate table size?

Hello, we are running Standard Editioin 11.2.0.3.  We have a purchased software package we are testing that uses the database and we want to get and estimate of how much disk space would be required to run thru 40,000 transacations.  First, I used the query below to get the current size of the tables.  Then, we ran thru 50 transactions.  I then ran the query again to determine the difference in size.  The  increase was only 72K for all tables combined.  This seems low and we feel it should be higher.  So, we ran thru another 50 transactions and I