What: Shares of Boyd Gaming Corporation (NYSE:BYD) jumped 55.5% in 2015, according to S&P Capital IQ data, surprising in a year when most gaming stocks dropped.

So what: Boyd benefited from its domestic exposure. While that's been a weakness for the company in past years, in 2015 the U.S. gaming market, and Las Vegas locales specifically, grew while markets like Macau shrank.

That growth helped revenue grow slightly, but there was a lot of leverage to cash flow as cost-cutting measures took hold at Boyd. For example, in the third quarter revenue grew 3.3% to $546.3 million, but adjusted EBITDA jumped 20.5% to $163.2 million. Since EBITDA, a proxy for cash flow, is the gauge many investors use to value gaming stocks, there was a valid reason the stock jumped during the year.

Now what: The U.S. has suddenly become one of the strongest gaming markets in the world, especially with the Asian economy facing a very uncertain future. The question heading into 2016 is how much more EBITDA can grow without stronger revenue growth. EBITDA margin of 30% in the third quarter would put Boyd among industry leaders, meaning there's limits to margin expansion in the future. That said, Boyd is performing well in one of the best gaming markets in the world right now, a situation that's making it the envy of its competitors.