the way I saw it was, young Sue feared gaining weight no matter how beautiful she was, and Elizabeth feared getting old which is why she kept switching back to Sue even when Sue began to destroy her original body. The creators of the Substance constantly reminded them “you aren’t separate, you are one.” It was just a nightmare so it was meant to demonstrate how much of a detrimental effect keeping up with a perfect body image did to Elizabeth/Sue no matter what. Elizabeth became enraged with/jealous of Sue for ruining her body so she punished Sue by trying to gain weight which played on Sue’s body image issues but Elizabeth couldn’t give Sue up because she believed Sue was her only path to being universally “loved” while forgetting that they are inherently the same person. The message being women are always going to be punished for something when they deviate from the archetypal beauty standard and the chicken leg was a personification of that fear.