Hi, out of general interest I spent a couple of hours poking around this issue and can say that:
It appears to be not reproducible with the Opera's stable version 66.0.3515.44
Annoyingly, it's not 100% reproducible with the Opera's Beta version 67.0.3575.8. But it sort of becomes close to 100% reproducible at times -> I can even quit Opera, restart it, and visit the web page and the issue persists.
Is Opera's adblocker and or VPN turned on? If so, try with those off.
I've tried with those options on/off and haven't noticed any impact one way or another.
Is Opera's battery saver turned on and you're running on battery? If so, try turning the battery saver off.
I'm not running on battery (I never am).
.. disable hardware acceleration and restart Opera to see if it helps
I've tried with hardware acceleration on/off and haven't noticed any impact one way or another.
I'll attach a few screenshots of the 'issue in action', maybe some knowledgeable guy might even be able to figure out something ..
#1 the initial status
#2 the offending page opened
#3 the offending page resized to minimum -> interestingly memory allocation gets reduced (to normal or thereabouts)
Well, that's all, perhaps it might be worth for a developer having a look at it since it's a 'feature' that has appeared post version 66.0.3515.44.