America's big casino companies are rolling the dice far from the Las Vegas strip. Sin City's real gamble is in China.

The majority of the business for these "Las Vegas" companies now comes from Macau, the only city in China where gambling is legal.

Consider the Las Vegas Sands. Its famous Vegas properties such as the Venetian and the Palazzo now make up a mere 10% of its sales, according to the company's latest earnings report. It's a similar story for Wynn. Its Vegas operations drive less than a third of the company's entire business.

Even MGM Resorts, owner of the Bellagio, MGM Grand and Mandalay Bay, is China focused. Its Macau business now takes in more cash than those three Vegas casinos combined.

"Macau is larger than Vegas, larger than the whole U.S. gaming market," says Adam Fleck, an analyst at Morningstar Research.

Related: China's gambling mecca desperately needs to diversify

China's gambling mecca: With a population around 600,000, Macau is the fastest-growing city in the world, according to the Brookings Institution, and its casino industry is roughly seven times the size of Las Vegas'.

But China isn't a winning bet right now.

An anti-corruption campaign by the Chinese government, travel restrictions and a smoking ban on casino floors are keeping gamblers with deep pockets away from Macau for now. China also just reported its slowest year of economic growth in decades, adding to a "perfect storm," of bad events in Macau, analysts say.

The Macau slowdown is hurting Vegas veterans -- and investors in these companies. The stocks are down sharply in the past year.

"Wynn and Las Vegas Sands have been betting big on Macau so that's why the stocks have down lousy," says John Staszak, an analyst at Argus Research, who has a hold rating on Wynn.

Related: China post its worst growth in 24 years

Investor are losing: Although Las Vegas Sands' reported record profits on Wednesday, its stock is down 30% over the past year. Sands reported a double-digit drop in revenues in its Chinese branch. The boost in overall profits came from cost-cutting measures.

Wynn (WYNN) will report its latest results Tuesday, and it's not expected to be a good story. The stock is down 26% in the past year. Wynn's casinos in Macau make twice as much in revenue as its Las Vegas ones. That's why it's very bad news for investors that Wynn's Macau branch is projected to report a 25% sales drop for the fourth quarter, according to analysts estimates.

But as the saying goes, the house always wins.

Related: Reinventing Vegas: Clubs not Casinos

Analysts say that casinos in Macau will be a long-term winner. The Chinese government has only granted six gambling licenses to casino companies, which reduces the risk of future competition. Once the casinos offer more entertainment activities outside gambling, they will be become even bigger cash cows.

"There's a very long-term secular growth story here," says Fleck, who has a buy rating on Wynn. "We think that the [casino] names look attractive right now, look undervalued."