> I don't see why this particular iteration should all the sudden have problems with that.
Well there's larger and there's a lot larger. This particular iteration increased the gap substantially.
3500 mAh (GN7) >> 3000 mAH (GN5) ~ 2950 mAH (i7+) > 2750 mAH (i6S+)
Below article offers speculation either way: that both the design itself and push for larger batteries was at fault and/or that it is simply a manufacturing error although they seem like two sides of the same coin -- at the end of the day you can't get around the fact that more batteries simply take up more internal space and the larger that 'more' is the harder the engineering problem is. Giving the engineers a hard design problem and then not giving them the corresponding necessary time to develop and test their designs is ultimately what's responsible for the defect.
http://phys.org/news/2016-09-galaxy-recall-stronger-batterie...
South Korean experts suggested Samsung may have been so ambitious with the Note 7's design that it compromised safety.
"There was no choice but to make the separator (between positive and negative anodes) thin because of the battery capacity," said Lee Sang-yong, a professor at Ulsan National Institute of Science and Technology who worked more than a decade at LG Chem, a leading lithium battery maker. Thicker separators can improve safety but will not necessarily prevent all overheating issues, he said.
Doh Chil-Hoon, head of the state-run Korea Electrotechnology Research Institute's battery research division, said that based on the limited information provided by Samsung, he believes the push to increase battery power was part of the problem.
"Even with a small manufacturing mistake, if there had been enough elements to ensure safety, it would not explode," Doh said. "It is a roundabout way of admitting weak safety."
...
[Wayne Lam, an industry analyst at IHS Markit Technology] said he thinks the Note 7 battery problem resulted from weak controls in manufacturing, not a poor or unsafe design.
A spokeswoman at iFixit, which publishes repair guides for electronic gadgets, offered a similar view. "We don't think any internal design changes in the Note 7 are responsible for the exploding batteries—more likely just a manufacturing defect," IFixit's Kay-Kay Clapp said in an email.
Apple has tweaked hardware and software it developed itself to make iPhones use power more efficiently, while Samsung has increased the capacity of the batteries in its phones.
That can be done without increasing size by adjusting components or changing the production process, Lam said.
"You have two different trajectories, with Samsung packing in more energy density, versus Apple trying to trim it down by optimizing everything else," he said, adding that the two rivals are "constantly locked in this arms race of improving and one-upping."
Note though that those comments come before the replacements started exploding, something that implicates a deeper design flaw than just the battery production.