The phrase "cocoon shape" refers to something that is rounded and enclosed, similar to a cocoon that protects a caterpillar as it transforms into a butterfly. Full definition
Victoria Victoria Beckham Textured Wool Dress in Navy Fully lined Brand new with tags, never been worn From the AW13 catwalk collection UK size 12, true to size cocoon shaped with a belt at back, which you could fasten around your waist. (vestiairecollective.com)
A kind of cocoon shaped white cardigan is shown below. (fashiongum.com)
The short sleeve design is fabricated in 100 % silk velvet and features a soft cocoon shape and a tie neck. (whistles.com)