Possible fun weightswap AU shenanigans, but maybe during Cas' attempts to help his partners lose weight, his old, 900lb plus appetite starts creeping in again. As much as Cas knows this is a fresh start for him, it's hard to break old habits...
Fast forward a few weeks and Cas is still WAY way slimmer than both his previous weight as well as the weight Oli and Rachel have reached, but he is looking noticeably puffier... chubby even.
Cas turns in for the night, resolving to try harder to shed his own weight so he can keep helping his partners shed theirs until he wakes up the next morning...
And finds he's once again trim and built like he was when this all started!
He's relived at first... until he notices that, subtly yet still noticeably, Oli and Rachel have puffed up slightly over the night in turn...
Now Cas has to try even harder to keep his old appetite in check, knowing that if he too slips, he might accidentally binge Oli and Rachel into being too fat to lose weight... If he does notice in the first place that is