I have observed that even though the memory usage as reported by developer tools under "JavaScript VM instance" stay rather stable in uBO[1] even after a lot of memory churning operations[2], the figure reported in Chromium's Task Manager keeps climbing up after each of these memory churning operation, and forcing garbage collection does not bring the reported usage down in the Task Manager.
There is something happening in Chromium's extension process (and outside the JavaScript VM in which the extension run) which may cause wildly various memory figures for even the same extension depending on how much memory-churning operations have occurred -- I wish Chromium devs would provide technical insights about this.
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[1] Around 8 MB when using default settings/lists.
[2] For instance, repeatedly purging all filter lists and forcing an update.