The story comes from reddit user doctorkuddles, who has a couple bits of interesting evidence to support the idea. The doctor says he recently purchased himself a 128GB iPhone 6 Plus, and compared it to his wife's 16GB iPhone 6 Plus, which was purchased soon after launch back in September. He notes a few differences between his device and his wife's, including simple tapping on the case to note the difference in sound, and even pulling out a stethoscope (he is a real doctor apparently) to listen to the devices and found his model sounded "much more dull toned" and "solid" compared to his wife's, which "had a lot more percussion sound, indicating that area was previously hollow".
He then put the device under a microscope and found a "red colored object placed in the volume down button crank". And, he even put it on a scale and found that his model was 21g heavier than his wife's device. The last thing he did was contact iFixit directly for confirmation. The team at iFixit bought a new iPhone 6 Plus and found it to be 0.8g heavier than the launch model that it had first looked at, which is well within the variance that Apple lists for its products. Apple says on its site that device weight can vary up to 1.5g from the listed official weight.
This could be something of a one-off error for the doctor, or it could be that there has been a change to the manufacturing process, and the stock hasn't made it around to all stores yet. Or, it could well be that the doctor made up the whole thing. At the very least, it seems like something to keep an eye on, because it would be very much like Apple to quietly fix this issue without saying anything or directly addressing any customers that have already had the problem.
