It came from defective batteries not thinness.
To me, that has to do with slimness.
The big problem with the Note 7 was the battery design and manufacture. Nothing within the device itself was the problem. These batteries would have exploded in any device they were placed in (though they were built specifically for the Note 7, like any phone battery nowadays): https://news.samsung.com/global/infographic-galaxy-note7-wha...
Samsung requested a battery with the design of the phone in mind. That design was faulty. Under normal operation the electrodes would expand and due to the design of the electrodes and the faulty separators, there was a fire.
The slimness/design of the phone necessitated a battery design which was faulty. I'm not saying that the battery literally had no extra room. There was no extra room without causing a short circuit and fire.
it's not a slimness issue