Weight gain is more common with TNF inhibitors, where we know that proinflammatory cytokines (eg TNF, IL1) in excess cause anorexia and wt loss and thus the use of a TNF inhibitor returns the pt to their normal appetite and potential for wt gain with control of inflammation.
Weight gain is not mentioned or hardly noted with commercially available JAK inhibitors. If weight gain does occur, I believe if would not be a direct drug effect but instead control of inflammation leading to a return of appetite. This is my understanding and opinion.