ELI15: before, when you wrote to disk, you needed a stronger magnetic pulse to change the polarity of the bit. This meant a wider area per bit. Now, we can heat a smaller part of the disk, making it easier to flip the polarity with a smaller magnetic pulse, so the area of the bit doesn't need to be as large, so we can store more info.

From TFA:

>The Exos 16 TB hard drive using HAMR technology is now the world’s biggest HDD in terms of capacity overtaking the 14 TB Barracuda Pro.

>HAMR, which is the acronym for heat-assisted magnetic recording, to be precise. This replaces the regular PMR, perpendicular magnetic recording, found in most HDDs. To the average consumer, this doesn’t mean much at all. However, Seagate believes that HAMR is the key to making significantly larger capacities readily available shortly.

That is a great question :-)