You can manually set the memory cache in Preferences.

I figure that unused RAM is wasted RAM. If I've got 4GB of RAM and all that caching allows to browse faster (going back a page in Opera is lightening fast), then I applaud it.

I like getting downvoted for not knowing the minutia of a program's memory handling. Thanks, people.

To Osiris: Given I'm not aware of that 150+ MB being dynamic, I assume that's 150MB I'm not getting back if and when I need it. I play memory-eating video games and use VMs; 150MB for a web browser looks like a lot when you've got to trim the fat off your running processes.

Does Opera reduce its memory footprint when you are low on RAM?

A random number like that is pretty useless. Opera has dynamic handling of RAM, so if you have a lot of it, it will use more. If you have less, it will use less.

I'm guessing you have at least 1-2 GB RAM.

By the way, claiming that Firefox is lean because it sets hard limits instead of adapting the memory usage dynamically is pretty silly...