This globe is not depicting where nuclear attacks have happened. This is stating where nuclear detonation testing has occured. The larger the nuclear cloud, the more testing has occured in that area. It just so happens that Nevada and New Mexico were heavily used for testing and thus the nuclear cloud is very large and extends to California.
If it was based on actual sizes of the bombs used our clouds would not be as large, although there would be a huge cloud sticking out of the northern part of Russia on the island of Novaya Zemlya where the largest nuclear detonation has ever occured (one which man created that is) which was around 50 megatons. The Fat man which only came in about 20 Kilotons was the one dropped on Nagasaki, to put it in perspective the fireball from the 20kt explosion was around .2km in diameter. The 50MT blast would have a diameter over 4.6km, for those not familiar with the metric system 4.6km is 2.8 miles and .2km is about .12miles. Keep in mind this is the fireball size and not the actual blast radius or area of fallout. An “average” nuclear weapon today is in the Kiloton range as advances in targeting tecnology has advanced and the need for precision engagment has increaed. The need for a cover all approach has been somewhat diminished.

As you can see if you read the reasoning behind a display you can make informed decisions. Now you know, and knowing is half the battle.