A green Christmas, a fat graveyard. The answer lies in the fact that a mild winter does mean life, but it is also life to the unseen world of deadly microbes and disease. However, a cold winter makes the world a veritable dessert. It effectively removes water from the environment.
Nothing can live without water.
Microbes either die or hibernate. The absense of microbes greatly reduces human mortality, i.e., a white Christmas makes a thinner graveyard. As in English this proverb has implied the verb. But, the Irish word order is usually the exact opposite of English when it comes to nouns and their modifying adjectives.