adverb taking the 'to' particle
Describes something pleasantly soft, swollen, or full in shape, such as bread, cheeks, or cushions. Often used with と or した.
See also: ふっくり
The freshly baked bread is soft and fluffy.
The bread baked up soft and fluffy.
The baby's plump cheeks are cute.
The baby's plump cheeks are adorable.
This rice cooks up nice and fluffy.
ぺちゃんこ describes something flattened or squashed, the opposite of the soft, puffed-up fullness of ふっくら.
Mimetic word; the exact origin is uncertain, but it is related to the sound-symbolic root ふく- suggesting swelling or softness.