It's because it's several times larger than you would naturally think it is, thanks to perspective tricks. It's the same principle that Murata
used in this panel, to make it look like Bang and Bomb are not that far away from the hole, when it's actually a few blocks away. Bang and Bomb are on high ground and the shot is angled so that we don't realize this at first.
In the first shot of the devastation with Iaian, it looks like the crater is only a block wide because the near side (where Iaian and AS are standing) is in the foreground and the far side appears a good ways back, creating the illusion that we can actually see the far-side directly across from Iaian and AS. But we can't, and the crater is a decent portion the size of the kilometers wide hole on the right. Using this perspective, you get a width of 1200 meters~
In the shot of the devastation with Homeless, it looks like Homeless Emperor is standing right in front of the crater because the POV is angled below him and the rubble in front of him looks much shorter than it actually is. On the right hand side, we see a shattered building near homeless, a two story chunk further up the slope and then a three story chunk near the top. Scaling here you get
a width/depth of 150 to 300 meters depending on which building you scale to.
So if you scale to the landscape, it's several 100 to 1.2km wide and many kilometers long. With vaporization, that gives crazy results. After all vaping a decently sized building is town level energy.