That's not a rule, e.g. big(er, est) has only one syllable and important(more, most) has several. Equinox◑ 14:04, 22 November 2011 (UTC)
Those are the irregular exceptions. The pattern followed by most one and two syllable word is red redder, reddest; and yellow yellower yellowest. With more than two syllables we have vermilion, more vermilion and most vermilion.