Users have blamed the iPhone 6 Plus bending non-scandal on dozens of things, from the weak aluminium case to the crushing force of their own arses. But there's a very simple, structural reason the 6 Plus bends, and thankfully, someone has explained it on Imgur with moment diagrams and everything.
A user by the name of alleras4 took to Imgur to speculate that the body of the 6 Plus has nothing to do with the bending—and their argument actually makes a lot of sense. They argue that the bending really has to do with a poorly-designed reinforcement inside the phone and behind the volume buttons. If you take a look at the reinforcements that line the phone leading up to the buttons, you'll see there are plenty of stable inserts to protect the body. But by the volume buttons, that reinforcement ends with a screw, leaving the body prone to stress.
"This means the metal insert isn't able to absorb moment so close to the screw because it doesn't have any other substantial point [to] hold on to thus it spins following the bending," says alleras4. Here's the view from iFixit's teardown:
alleras4 explains that this is where the highest stresses accumulate, and he or she illustrates this using a moment diagram, above, which is a simple tool to draw where forces (in this case, your hands/bum) come down on a beam (in this case, the 6 Plus) and result in the largest moments (the points most prone to deformation, or bending).
In other words, it has nothing to do with strong thumbs or weight. It has to do with hitting the 6 Plus in a very specific place, from a specific angle, where Apple's internal hardware design has a flaw:
[I]t's not about if it bends or not, as seen in the video we know it does where other phones don't or less so. It's not about how much force must be applied and if a pocket will do the trick or not. It's just that under a particular type of flexing, the phone is prone to bend mainly because a metal insert meant to reinforce instead spins in an axis too close to the critical point. If they were further apart allowing better support to counter the flexing and not spinning, it would make it more resistant.
Of course, as alleras4 points out, this is based on simple structural logic and the teardown—but it's pretty convincing. So there you go: Wedge your iPhone into your tightest jeggings with impunity. Just don't bend it at this one spot. [Imgur; Ifixit]