The usageagent.log file is automatically rotated when its size reaches the value of the LogFileSize preference (which is typically 4MB by default). However this rotation only happens when the usage agent is started. Because the usage agent is often a long-running process, the log file can grow substantially larger than the LogFileSize limit by the time the agent is restarted.
This issue can be worked around by forcing a restart of service running the usage agent. This will also occur when the operating system is restarted.
See the following articles for other issues related to usage agent logging: