The conjunction rule of probability theory states that a conjunction (A and B) is necessarily less probable than one of the conjuncts alone (A). Adding more details to a theory may make it sound more plausible to human ears because of the representativeness heuristic, even as the story becomes normatively less probable, as burdensome details drive the probability of the conjunction down (this is known as conjunction fallacy). Any detail you add has to be pinned down by a sufficient amount of evidence; all the details you make no claim about can be summed over.

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Burdensome Details

See also

Conjunction fallacy
Representativeness heuristic
Occam's razor, Epistemic hygiene