(There is probably a more grammaticaly correct way of explaining this, but...) The particle cyn here is used to introduce a particular form and meaning of an adjective. mor can also be used in a similar way. You can think about them acting as a link between dw i, etc and the adjective rather as does yn/'n - you don't need both yn and mor/cyn.
Note that cyn gets embedded with a few forms of adjectives, as above. mor does not do that.
Note that the words cyn, mor also have other meanings and uses.