Fiddler version: v5.0.20204.45441
If Fiddler is left for some time to work unattended then its memory usage is constantly growing.
I usually use Fiddler with some Autoresponder rules enabled and "Unmatched requests passthrough" option switched on. If left in this state for several hours an amount memory it uses could grow up to 10GB and even more which is quite annoying...
Autosave option is enabled and sessions are dumped every 5 minutes. So this behaviour is not because of session list is becoming large, it is being cleaned every 5 minutes.