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Opera seems to start off light on my system, and with each page load the memory usage slowly starts to climb, it never goes back down. Perhaps Opera has a memory leak bug in it.
No, this is memory cache feature. You can disable this behavior in preferences - network and caching.
(When you have a lot of free ram Opera freely allocates it for caching web pages, so you can have instant 'back')
It is known that Opera slowly starts/quits when you have huge global.dat file (history of visited addresses, you can shorten it in prefs, too).
It has already been fixed in latest beta (unfortunately unstable yet, available on my.opera.com/forums)