That shape has absolutely dick to do with anything but the size of the explosion and the amount of moisture in the air when it went off.....

the cloud you're seeing here is not at all like the ones you're comparing it to in nuclear explosions.

If you watch a bunch of nuclear explosions sometimes you will see some white fluffy clouds moving outside of the main mushroom cloud. THAT is what you're seeing here. What you're comparing is apples to oranges.