If West did gain weight (and I strongly suspect that it's just the cut of his shirt, BTW), it's most likely due to the medications he takes to help with his mental health. He was hospitalized in November of last year for "exhaustion and sleep deprivation," and was likely given medication. One of the side effects of medication? Weight fluctuation.
And no matter what, fat-shaming is never funny. It's just plain cruel to make fun of the way someone's body looks.