Normally it would be something like "jmd. eine reinhauen", but that doesn't fit here, and it becomes even more difficult with the comparition in the end.
(Isn't that unclear in the English sentence, too? The hole is just 'as large as the arsenal', not as large as a hole made by the arsenal's explosive force)
"... schätzt, daß der Einschlag eines Kometen oder Asteroiden ein Loch machen würde, das 15 Mal größer wäre, als alle Atomwaffen der Welt zusammen [es könnten.]"
Robocop's translation sounds nearer the mark to me. I, too, think that we're talking about the quantity of energy here, rather than the size of the would-be crater. Hence the potential ambiguity in English pointed out by Lykurg doesn't exist from my point-of-view.
As another possible variation, how about ...?:
"... schätzt, daß der Einschlag eines Kometen oder Asteroiden eine Schlagkraft hätte, die fünfzehnmal größer wäre, als die des gesamten Atomwaffenarsenals der Welt."
(Hope I haven't made any grammatical mistakes there. It's a pretty tough assignment as far as punctuation and gender/case agreement in German goes! )
Of course, however one translates it, it's a pretty silly statement that has little or nothing to do with reality (but which is probably fine for the comic/science fiction world that spohreis is interested in ). In particular, the factor surely depends on the size of the comet or asteroid, and probably no-one really knows how big the global stockpile of nuclear weapons is, either!