On web servers, and Exchange OWA servers especially, there are a lot of OWA logs which can be found in C:\inetpub\logs\LogFiles\W3SVC1.
This small Powershell script can be useful, it will delete IIS Log files older than 60 days:
get-childitem -Path C:\inetpub\logs\LogFiles\W3SVC1 -recurse | where-object {$_.lastwritetime -lt (get-date).addDays(-60)} | Foreach-Object { del $_.FullName }