puppy fat vs fatty tissue

puppy fat and fatty tissue both are nouns.

Word	Noun	Adjective	Verb	Adverb
puppy fat	Yes	No	No	No
fatty tissue	Yes	No	No	No

As nouns, fatty tissue is a hypernym of puppy fat; that is, fatty tissue is a word with a broader meaning than puppy fat:

puppy fat: fat on the body of a baby or child; disappears at adolescence
fatty tissue: a kind of body tissue containing stored fat that serves as a source of energy; it also cushions and insulates vital organs

Other hypernyms of puppy fat include adipose tissue, fat.

puppy fat (noun)	fatty tissue (noun)
fat on the body of a baby or child; disappears at adolescence	a kind of body tissue containing stored fat that serves as a source of energy; it also cushions and insulates vital organs