Recent versions of opera are not releasing unused memory. I just managed to terminate opera 24.0.1558.64 which had 4 tabs and it was using 3 Gb . I restarted opera and it used about 1 Gb . My machine is a 6 Gb celeron on windows 7 64bit.
I have been having performance probs on opera , both linux & windows for 1 or 2 years particularly on Google Mail . I have used opera for 10+ years but now I have to use something else