777's on the route with 250 bums on seats will turn a profit (a guess) and can run more regular flights if the numbers warrant and can be intermingled with domestic and other international ops. An A380, that can't be used domestically and has limited integration on other international flight ops, with 250 BOS won't be on the route for long.
Not defending Delta, but if the domestic market is slowing, new international routes utilising excess capacity makes sense. What's Qantas intending to do with the A380 if the market continues to tank? One or two flights per week won't compete against a daily service.