Yes, it's true.
The MGM Grand has 5,033 hotel rooms. If you add in the 576 rooms at MGM Residences, it comes to 5,609. Note, however, that since the MGM Grand Hotel-Casino and the Residences aren't connected and are co-owned by Turnberry Associates, technically they're not part of the same hotel.
The First World Hotel, located in Pahang, in the Genting Highlands of Malaysia, has 6,118 rooms. Opened in 2006, it's one of five hotels located at Genting, which you reach by taking the Genting Skyway, the world's fastest (and Southeast Asia's longest) cable car, which travels at 20 feet per second. The First World exterior is painted in a kaleidoscope of psychedelic colors.
Along with its more than 6,000 hotel rooms, First World has the 500,000-square-foot First World Plaza, with the Starworld casino, a shopping mall, arcades, food outlets, entertainment venue, and an indoor amusement park with four roller coasters and Sky Venture (Asia's first free-fall simulator), a Winter Wonderland feature with indoor tobogganing, and an indoor water park. There's also a golf course, 28-lane bowling center, and replicas of world landmarks.
(First World receives 2.5 stars out of 5 based on 46 reviews on TripAdvisor.com; many people complain that it's too big and out of control, taking two-three hours to check in, with unfriendly employees, small rooms, and "palatable" food.)
If all goes according to plan, First World won't be able to make the world's-largest-hotel claim for long. Under construction is the 6,500-room Asia Asia Hotel, part of the massive Bawadi project at Dubailand in Dubai, scheduled for completion in 2010.