In HTML5 <big> is deprecated, so I do not see any reason to have a button for it in the toolbar. The probably only reason why it is there is consistency with <small>, but since HTML5 isn't consistent in that point the toolbar needn't to be either.
Version: unspecified
Severity: normal
URL: http://www.w3.org/TR/2012/WD-html5-diff-20120329/#obsolete-elements
See also