Galbatorix’s **** Explosion Yield Issue

Just made it through the part where Galbatorix commits toaster bath by deciding to change careers and become a nuke.

Being a physicist or indeed just anyone who's seen E_0 = mc² before, I found it weird that Galbatorix's total rest energy wasn't enough to atomize Urû'baen and several hundred kilometers of the surrounding countryside.

After a quick back-of-the-envelope, my suspicions were confirmed. A 62kg mass getting converted into pure energy would give comfortably over a gigaton in yield. This is enough to resolve most issues in Alagaësia by way of no longer existing.

I don't recall exactly how they described the spell "be not" earlier on, but I definitely didn't hear it described as "be not gradually". Did they ever explain why the total mass energy didn't contribute to the resulting blast?

I recall that the same thing happened on Vroengard since:

1 - the island still exists

2 - the area is still heavily irradiated which means that it was a very dirty bomb.

EDIT:

Thank you all for the responses. I generally like the idea that "be not" just converts parts of the body into energy. I was thinking about it, and maybe it was just the part causing the most pain, so some portion of his mind perhaps.