Depends on the documents and how many there are. On your typical 50-100MB web-appy page (e.g. gmail) a couple MB is a few percent. On about:blank, which is no

Depends on the documents and how many there are. On your typical 50-100MB web-appy page (e.g. gmail) a couple MB is a few percent. On about:blank, which is normally a few hundred KB, a couple MB is several thousand percent. So if a page has lots of subframes and those subframes don't have much in them, you get large wins. The canonical things that have lots of subframes are techcrunch and the like, which have "like" buttons for several different social networks and whatnot all over them. Each of those "like" buttons is a separate iframe. https://bugzilla.mozilla.org/show_bug.cgi?id=988266#c7 https://bugzilla.mozilla.org/show_bug.cgi?id=988266#c7 has some relevant numbers for techcrunch; the difference there was 300MB vs 520MB or so (which means that the page has about 100 iframes, of course).