The point I've tried to make in other posts is that what you describing isn't what we're seeing.
What's on screen isn't multi-megatons or even several kilotons. It looks like the explosion from the equivalence of tens of tons of chemical explosive. Even with a focused detonation, super tough hulls, shields that absorb energy, a warhead the equal the the biggest old russian bombs (58 mt) would produce a ball of energy in space several miles across.
A isoton could be like the British Thermal Unit - BTU - a standard unit of measure that extremely small.
It begs the question, why are starfleet and their enemys producing "little warheads"?
The thing about post-TMP "on-screen treknology" is that not one bit of it is remotely to anything resembling "real scale."
In real spacecraft interactions, you'd never have two ships "bumping-against-each-other-close." If you could see a tiny dot of light in the distance, you'd be almost "too close."
Believe it or not, this is borne out by the WORDS used in latter-day Trek (say, TNG). They'll state that two ships are "2 million kilometers apart" yet in the SFX shots, both are visible. (Which would mean that those are some pretty freakin' huge ships... far larger than anything Abrams could've come up with!)
SO... if the words in the scripts differ entirely from what we see in the SFX shots, and if the words used in the scripts make sense while the SFX shots don't... we have only one real choice. That choice is to assume that the SFX shots are "stylized representations" of a reality which, if we were watching it, would be utterly boring to watch.
Now, I've got my own "personal conceit" as to how this works. See, I absolutely and utterly reject any suggestion that the "main viewscreen" is a window, or remotely resembles a window. Rather, it's a computer monitor, and what it shows is intended to provide useful information, not visual accuracy.
So, it creates "3D iconic representations" of any ship it's intending to display, but dramatically overscaled so that they're visible, and adjusted (remember, deep space lighting is almost nonexistant, thus you wouldn't see anything anyway most of the time) for "studio lighting."
The ships are scaled waaay up so that you can see them. And what we see on-screen is "scaled" in the same fashion... what we see are "viewscreen images," not "realistic in-space images."
What does this have to do with the topic at-hand?
Well, the explosions we see on-screen would also be "scaled for usability" rather than shown realistically.
Remember, the original Enterprise had the capability of wiping out the entire surface of a planet... and in fact this was something that most "class one starships" would be able to do. Hell, they even had a general order to regulate how this would be done. That's a hell of a lot of firepower for a ship of that size. You need massive weapon yields to accomplish that.
But... a couple of dozen 1-isoton warheads could accomplish this quite nicely, I think.
No... just accept that the on-screen SFX are "stylized representations" and that the reality is much more in-line with reasonable reality and in-scripts commentary.