An Asian/South Pacific cruise is so much more expensive than even a European cruise - plus the flight costs are greater. Since this type of cruise is so much costlier, I think a smaller ship would be more likely to sail this type of itinerary with a greater occupancy rate than the new larger ship.
Celebrity currently has the Century and the Constellation doing basically the same ports and the Millie and the Galaxy with the Century for part of the summer doing the Med with most of the same ports. For the Med cruises, most of the ports are the same on all the ships.
Millennium-Transatlantic-9/01, 10/03, 11/05
Zenith-Special Caribbean 11/06
Back to back Millennium 28 days - Mediterranean 11/18/07, Transatlantic - 12/2/07