Him being big has nothing to do with being an effective pitcher. he's also not "praying with power throws" when he's never been a power guy. His fastball is at or slightly below league average.
His biggest issue is that he's lost velo from last year, his slider has lost break from last year, and his performance last year was in part significantly backed by unsustainable luck (as well as benefits that have been lost with the shift ban) that was bound to normalize and hurt him.
He does have some things to sort out but it has less to do with losing weight and more about figuring out why the stuff that made him effective (aside from the BABIP luck and strand success) have regressed or deserted him this year. Maybe it's an injury. Maybe it's a mechanical adjustment. Hard to say at this point.