I don't think there's anything unexpected about this difference .... Epiphany uses the "document font" set in gnome control centre fonts applet (unless you specify your own font in Epiphany's preferences), which defaults to Sans 10 (that's 10pt). Firefox defaults to 16px.
-- Different font size compared with Firefox https://launchpad.net/bugs/43837 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs