Sorry if you already know some of this, but if not hopefully it's of some help.
Domain Authority isn't calculated based on your website content's quantity or quality, it's based on link metrics of other sites linking to your website.
"We calculate this metric by combining all of our other link metrics (linking root domains, number of total links, mozRank, mozTrust, etc.) into a one single score" (http://www.seomoz.org/learn-seo/domain-authority)
Run both domains through the Open Site Explorer and compare the number of links from root domains + the domain authority scores of those links and you should begin to see the difference.
It's probably also worth watching Rand's Whiteboard Friday on Domain Trust & Authority: http://www.seomoz.org/blog/whiteboard-friday-domain-trust-authority