Are d20 Future HP comparable to regular old D&D 3.5 HP?

So, for research reasons, I am interested in the explosive yield of the SRD’s Hellball with its 40d6 damage over a 40′ radius. However, I have come up with two wildly disparate results based on the path taken, so I believe there’s a logical issue somewhere, but my math pointed me at an odd result, namely that d20 Future uses a different "scale" of hit point than a regular fantasy D&D 3.5e game would. Is this correct, or have I gone awry somewhere?

Path 1: conversion-by-damage-dice

As pointed out by Hey I Can Chan in an answer to a similar question, the Nuclear Missile from d20 Future, with its said-to-be-1MT warhead, does 16d8 damage, which is roughly equivalent to 21d6 according to his translation of the damage dice. Extrapolating linearly from this datapoint puts us at a Hellball yield of ~2MT, which is certainly hellish enough!

Path 2: conversion-by-effective-radius

However, to try to cross-check this answer, I took the 40′ damage radius provided by Hellball, mapped it to the "nothing survives" fireball radius of a nuclear warhead, and took the NUKEMAP for a spin. However, even with a surface blast from the tiny 20ton Davy Crockett, I get a fireball radius of 20m, or over 60′; getting a smaller fireball than that requires going to a 10-15ton yield, although model uncertainties become limiting at this small scale.

Of course, if you try to plug the Path 1 answer of 2MT into NUKEMAP, you get a far larger fireball of about a mile wide. Furthermore, 16d8 of raw energy damage sounds like a lot…but in the context of anywhere-close-to-upper-level D&D 3.5 characters, even with no saving throw, resists, or SR, the 128 maximum damage it produces is quite tankable. For example, the character that prompted this inquiry has 220HP with only their gear warding them, and still manages 112HP stark naked.

As a result, I am sitting here wondering if I’m barking up the wrong tree somewhere.