Shared Virtual Memory Usage Question

We have a nice, simple two tier application, with a Powerbuilder based client connecting to a V7.13 engine. New connections start out using very little memory (virtual pool I think) as reported by "onstat -g ses". Then over time it increases, some getting as high at 20+ megabytes. Usage is mostly OLTP, some light reporting also. Anybody have a clear understanding of what kind of client activity uses this memory over time (it seems to be freed up when the session is terminated)?? Greg