It wasn't until around 2010--with the release of the first Galaxy S and Droid X--that the market had some legitimate iPhone alternatives. Manufacturers seeking to innovate beyond Apple hit on a new insight: iPhone's polish aside, people were desperate for more screen real estate. The rest, then, is history.
There's probably a kernel of truth to this, but the theory is too neat. Both Google and Apple have copied regularly from one another.
Scenario #3: The changes are largely based on a handful of influential phones
A more nuanced version of Scenario #2 suggests the smartphone's increasing screen size resulted from a handful of iconic phones, each of which increased screen size and each of which created a mini-revolution.
This makes some sense: There might be more than 1,000 phones in the market, but only about half a dozen per year dominate headlines and sales. With each successful change (examples: the iPhone 4's sharper display; the better cameras on early DROIDs), other marquee phones have quickly followed. The same is probably true of screen size.
Would something over 4 inches work? The DROID X proved a "big" (for its time) phone could be successful. But what about over 5 inches? The successful, 5.5-inch Galaxy Note II unleashed a whole series of similarly giant phones.
For the most popular phones in the industry, the "trendsetter theory" probably explains a few shifts in the market. But it doesn't fully explain the dramatic, macro-shift we see across the entire industry.
Scenario #4: Manufacturers have always wanted to make bigger phones--technology simply hasn't allowed it until recently
In 2007, both pixels and battery life came at a hefty premium. Trying to power a 5-inch display with a reasonably high pixel count just wasn't a possibility. Today, battery and display technology allow manufacturers to make crisp, 6-inch-plus screens that run for well over a day.
Just look at how pixel densities and battery capacities have scaled in Samsung's Galaxy S line across five generations:
It's the simplest explanation, and perhaps the best one so far. It helps explain why manufacturers wouldn't touch designs bigger than 4 inches before, but now they churn out 5-inch-plus models routinely.
It does not, however, explain why manufacturers have all but abandoned sub-4-inch phones since 2013...
Scenario #5: The smartphone is turning into our primary computer
Even in 2007, it wasn't yet clear that the smartphone would become the staple product that it is today--the sort of device that could one day replace most personal computers. Seven years ago, the smartphone was still a combination of three less significant products: a music player, a mobile web browser, and cell phone. Today, the smartphone connects people around the world like nothing before it. Citizens of third-world countries are unlikely to own cars and computers, but they are rapidly buying smartphones.
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