3 倍でしょうか？

「省メモリで選ぶならFirefox 3.5、Chromeと3倍以上の開き」のはてブコメントにも書きましたが、みなさん具体的数字がお好きなようです。

If you're measuring memory in a multi-process application like Google Chrome, don't forget to take into account shared memory. If you add the size of each process via the Windows XP task manager, you'll be double counting the shared memory for each process. If there are a large number of processes, double-counting can account for 30-40% extra memory size.

や、これを引いている以下の記事には Opera への言及もあります。

Opera is using an automatic setting called Automatic RAM Cache that is enabled by default. This allocated about 10% of the computer memory by default and should be disabled in memory benchmarks. Opera would most likely still have been beaten by Firefox but the gap would probably have been lower.