This chart shows unit sales of each products in the first six quarters after launch. The iPhone launched in June 2007, but didn't really take off until the iPhone 3G launched in July 2008, five quarters later.
The iPad launched in January 2010, and has sold more units every quarter.
That's partly because the iPad benefits from all the work Apple did for the iPhone, including a huge base of developers and customers who already understand how the product works. The iPhone is also constrained by carrier contracts -- it's hard to convince customers to buy a new phone before their current contract is up -- and had stronger competition at launch (like the BlackBerry).
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