It's known that Opera 21 is using a little bit more of RAM than 20.
Have you tweaked any opera:flags? You should consider resetting them to default and observe the RAM usage.
Disable plug-ins you don't use in opera:plugins.
And instead of following @lem729's suggestion of installing the extension you may want to try an automated solution Opera Software is developing called "tab hibernation". You can enabled it in opera:flags and read more about its technical details and other options here.