Reduplicated adjective from 鼓 (drum, but verbally "to bulge/swell"). Almost always used with 的 attached: 鼓鼓的 X. The fuller form 鼓鼓囊囊 (gǔgǔnāngnāng) intensifies the "bulging-and-stuffed" feel — bag, pockets, cheeks all qualify.
memory
鼓 is literally a drum, and a drum's stretched skin bulges outward — a perfect mnemonic. Reduplicate to make it vivid: "drum-drum-y" = bulgy, puffed out.
