The “top talkers” feature tracks the highest-bandwidth consumers in your network. Lowering the optimization value means more flows and more detailed data are collected and stored, which can increase the storage requirement on your server.

Exact Extract from Official Guide:

“Decreasing the Top Talkers optimization value results in the storage of more detailed data, which increases the database and storage requirements.”

— (Network Performance Monitor Administrator Guide, “NetFlow and Top Talkers Optimization” section)