The translator chose to "translate" the Russian measure of weight of that time into something that would make sense to English natives.
In the original text Dostoevsky uses the phrase _семипудовая купчиха_ , where _пуд_ is approx. 16 kilos, while _семь_ means "seven".
The narrator ("devil" from Ivan's nightmare) alludes to a stereotypical image of a "merchant's wife" ( _купчиха_ ), usually depicted as.. well, a couple of examples can explain it better:
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