It's possible that, for your largest network, you are encountering SAS memory limits that can be adjusted via use of general SAS options such as MEMSIZE=. On what operating system are you running? What does the log from your SAS call for the 7.5M arc network look like? I'm interested specifically in the log messages produced by the MEMREP option that you're already using. These messages will tell us about the memory usage and about any paging that NETFLOW is doing.
If you like, we could take a look at your data and code and advise you--contact me at Ed.Hughes@sas.com.