640px is the exact width of the iPhone 5's screen. While you may want larger images, the fact is that Instagram has a large number of users in foreign markets

640px is the exact width of the iPhone 5's screen. While you may want larger images, the fact is that Instagram has a large number of users in foreign markets and they're trying to expand their presence there. Increased resolution means larger resource requirements, and ultimately, a slower/reduced experience for the users they're trying to reach. To be honest, a community like 500px might be more along the lines of what you're looking for.