At Alphabet  (GOOGL)  unit Google's I/O conference beginning Wednesday in Mountain View, Calif., CEO Sundar Pichai gets the chance to play up new products. Pichai also has the opportunity to stake out Google's position in a global app ecosystem rivalry with Apple (APPL) , which will hold its own World World Wide Developers conference in San Jose in early June.

Apps are a fast-growing business for both of the Silicon Valley icons. Consumers spent $45 billion in Google's and Apple's app stores last year, BMO Capital analyst Tim Long estimated in Tuesday report, growing 36% from the prior year. The two paid out $31 billion of that to developers last year, according to Long's calculations.

BMO estimates that Google Play made $5.4 billion in fiscal year 2016 sales, accounting for about 6% of Alphabet's top line.

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Apple is still the top dog, however. BMO Capital calculates that the company's app store generated $7.3 billion in fiscal year 2016 revenue, or 35% more than Google. Still, BMO says that the app sales represent just about 3% of Apple's total revenues.

The iPhone was the original showcase for smart phone apps, giving Apple a head start on the market. In addition, Apple users tend to spend more on average on apps than Android users. 

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