Quite big, but not immeasurably so (8)

I believe the answer is:

sizeable

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'quite big' is the definition.
'sizeable' can be an answer for 'big' (similar in meaning). I'm unsure of the 'quite' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for sizeable that I've seen before include "Large" , "Largish" .)

I've seen this clue in the Evening Standard.

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