Such a lot is so little! (3)

I believe the answer is:

fat

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'such' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for fat that I've seen before include "Adipose tissue" , "Portly" , "Like 10 ac" , "Substantial" , "eg Lard" .)

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