They are simply using virtual memory as requested I suspect. If you run an identical version of Opera on a constrained device it will use a minimal amount of RAM. If you run the same rendering engine on a system with lots of free RAM, then it will use more RAM. The Opera browser is a great example of that, same rendering core from mobile phone up and code scaling as the OS allows greater allocation.
Chrome does use crazy amounts of RAM because of its process-per-page architecture, and I think is not a recommended browser if you open lots of pages simultaneously. Funnily enough Firefox (after years of criticism), is much more memory conservative than Chrome these days.