Personally I don't think the "growing" file would be growing for long; I'd rather expect it to be created (and reach the huge size) almost instantaneously - when those special (unknown) conditions are met... though I'm just speculating.
While the Process Monitor tool can be running on background for quite a long time, even if you set the filter to monitor only the specific path - starting with C:\Users\yongl\AppData\Local\Temp\_avast_\unp - the log can still be rather big and eat some memory (plus it's easy to forget to restart the tool after the machine is restarted).
But there are some interesting things here...
First that the file is in your user folder (and not in c:\Windows\Temp\_avast_)... I kinda missed it the last time, but it means it's not the main Avast scanning service, but rather something else (still don't know what though).
Second - no files should normally be left in those _avast_ folders, in my opinion. I mean, the folder is used for temporary storage - so files can appear there, but - except for a "lock" file - should be removed soon (well, unless something like a Windows BSoD happens... or powering down the machine without closing Windows first). Files that cannot be deleted are OK (they are still being used); files that can be deleted there are suspicious. If you say you've got 4 files there, 4 days apart... i.e. files two, three weeks old there - that looks like an improper cleanup (could be an unrelated issue though).
Still, without being able to reproduce (or possibly the Process Monitor log), there's little I can do I'm afraid.