It's about the excess overhead you have. An OS that uses anywhere from say 500-750 mb of ram on os boot on a device with 2 gb of RAM has over 1gb of overhead for apps or anything else.
If you have 512 mb on your device and the OS takes up 200 mb that's not much overhead for your apps or tabs. Even less when the OS takes up around the same amount.
but I thought iOS and OSX will use as much RAM as it has available even it it does not exactly "need" that much RAM to function and as it needs more it "reshuffles" RAM to be distributed where its needed?
My guess is Safari or how iOS makes new tabs is the issue, it freezes the tabs not in the foreground making them reload when you need them again if the tab requires more RAM