Normal RAM usage?

What is the normal RAM usage for Midori? For me it usually starts off at about 50-100MB, and slowly rises to 600-700MB after some time of browsing a few tabs and watching videos. This seems a bit high compared to e.g. Firefox. Closing the tabs doesn't make the RAM usage go down much.

Command line midori
Midori 0.5.9 ((null)) Midori
GTK+ 3.14.8 (3.14.8) Glib 2.42.1 (2.42.1)
WebKitGTK+ 2.4.7 (2.4.8) libSoup 2.44.2
cairo 1.14.0 (1.14.0) libnotify 0.7.6
gcr No granite 0.3.0
Platform X11; Linux x86_64
Identification Mozilla/5.0 (X11; Linux; rv:2.0.1) Gecko/20100101 Firefox/29.0.1 Midori/0.5
Video Formats H264 [x] Ogg Theora [x] WebM [x]

Extensions enabled: Ad blocker
Javascript enabled
WebGL disabled