If you generate a report daily or weekly is it ok? Did you try to rebuild the logs?
Did you set something on the System Settings -> Advanced -> Device Log Settings: Roll log file when size exceeds x MB and the Roll log files at regular time ?
We are running FAZVM with 5.0.7 and I have the size for log files 200MB set and the weekly roll log. Maybe it will help? (I am not sure what is the default).
What is the CPU and Memory usage during the report generation? Do you have some errors in the System Settings -> Event log?
NSE 8, CCNP R+S