[Old Professer]

And the actual blast wave is from the superheated air in the tank which, even when full, is 20% or more of the total volume.

[ArrogantBustard]

I seem to recall reading that the BLEVE is most likely to occur when the tank is between 1/2 and 3/4 full; the "Expanding Vapor" comes from the "Boiling Liquid". Of course, if the contents are flammable, you'll also get a fireball if there's any source of ignition.