Memory not getting freedup when the sessions are open

Hi: We are running 1.02UC2 newera & Online 7.14UD2 on AIX 4.1.4. We have about 30-50 users. Each session sometimes uses upto 20MB of memory (shown in onstat -g ses). This memory is not getting freed up. We tried onmode -F. But this did not do any good. I heard that Informix keeps the memory for each session as it uses it, assuming that the user is likely to ask for more. Is this true? If so, is there a solution to our problem other than kicking out the users every night? FYI, we verified our client code to see of any open cursors etc. nothing was found. thanks in advance, - Reddy