Edit the Microsoft.Exchange.Diagnostics.Service.exe.config file in the bin folder by adding the following line in the appSettings area:

<add key=“MaxDailyPerformancelogDirectorySize“ value=“1024″/>
or lower <add key=“MaxDailyPerformancelogDirectorySize“ value=“512″/>

The value is the maximum space consumed for daily performance logs, in megabytes.

Then restart the MSExchangeDiagnostics service for the change to take effect.