The young blubber? (5,3)

I believe the answer is:

puppy fat

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

(Other definitions for puppy fat that I've seen before include "Adolescent flab" , "Early chubbiness" , "Childhood tubbiness, often lost by maturity" , "Childhood plumpness" , "Adipose tissue disappearing in adolescence" .)

I've seen this clue in The Sun.

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