Is this true? Why are all major product releases in the US if most of their revenue is coming from overseas?
Apple's US unit volume is close to 50 million per year. For 2014, Chinese consumers are expected to buy close to 300 million smart phones, and Apple has close to 15% of that market. So I'd say it's a pretty tight race currently. Apple has a big problem in China however, they're getting squeezed substantially by the domestics (Xiaomi etc). In the US that's not happening, and Apple has held on to their strong ~45% market share.
http://www.ibtimes.com/iphone-6-preorders-china-set-surpass-...
"China could soon surpass the U.S. as the biggest market for Apple Inc.'s iPhone. A report on China's Tencent Monday said Chinese consumers preordered 20 million iPhone 6 and 6 Plus smartphones in the first weekend they were available for preorder.
"If those numbers turn into sales when the iPhone is released Oct. 17, China will have doubled the 10 million phones sold in the first weekend in the U.S., setting it on the path to be the world's biggest market for iPhones."
http://www.washingtonpost.com/blogs/wonkblog/wp/2014/08/20/w...
In that time the US has added $3 trillion to its GDP, going from $14.5 trillion in 2007, to a present $17.65 trillion or so. It's likely the US has reclaimed its title as the largest economic zone. Given the US is set to expand at close to 3% for 2015, and is growing much faster than Europe, that gap is likely to expand in the next few years.