To understand what you're seeing, I'll need you to answer a few questions:
0. How much physical memory is on the system in question?
1. What version of Fiddler do you have? (Help > About)
2. What setting do you have for "Tools > Fiddler Options > If client aborts while streaming"?
3. In the scenario below, what is logged when you type !memory
and hit enter in the QuickExec box below Fiddler's Web Sessions list?
4. Do you have any 3rd party Fiddler add-ons installed?
5. Is there any data being logged to the LOG tab (e.g. mention of errors, etc) as memory usage grows?
6. What network-using applications does your system have? E.g. do you have DropBox/OneDrive/etc configured to sync your files?
One important thing to understand is that the .NET Framework doesn't necessarily perform garbage collection unless the system is under memory pressure; if you have gobs of free memory available, it's willing to use it. One trick to help induce Fiddler to release unneeded memory is to minimize it using the minimize button at the top right.
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