I have the same problem. Always have, for over two years of various Opera versions (currently I'm on 12.14). It even made me switch to Chrome for 6 months out of frustration (until Google got a little too big brothery for my likes).
Right now, I have 4 tabs open. Ebay, Gmail, a forum, and this Opera website. Opera is using 785,111K memory. That's a little bit ridiculous, right? When I shut down Opera (which takes 5 minutes, I might add, before the process finally disappears) and restart it, memory use is better but after an hour or so it's right back at hogging 1GB of memory. I have 4GB memory, a quad core and am using Windows 7 64-bit. The only other applications I run are a ten year old version of Winamp, an explorer window (not IE, just explorer), truecrypt and notepad. Of those, Winamp uses 3 MB of memory, Notepad 1 MB, Truecrypt 2MB, and explorer 37 MB. Yet somehow Opera just uses 800 MB and if I open more tabs it uses even more... note I have zero add-ons or plug ins. Just a regular version of Opera.