Spot of bother with group of animals -- half of shed is filled to capacity (3-6)

I believe the answer is:

jam-packed

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But I can't explain it at all! Could you help me understand?

(Other definitions for jam-packed that I've seen before include "Stuffed" , "Completely full" , "Heaving" , "Chock-a-block" , "Extremely crowded" .)

I've seen this clue in The Telegraph.

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