Because the brilliant witch wanted to stay skinny, she created a clone that would become a storage for her fat. The creation worked like a charm; the witch could indulge in whatever she'd want and stay as thin as a board, while her body double got bigger and bigger. This caused the clone to develop a weight gain fetish. Being an immortal creature there would be no limit to how big she could get, but it also meant she didn't need to supply herself with food. So in order to make sure she kept growing, she had put a spell on her master; a curse of insatiable hunger. As the witch ate endlessly, her intake far outpaced the speed at which her fat would transfer to her clone. When she finally ran out of funds (and clothes that could cover her new size) the witch being unable to eat finally had enough willpower to confront her behemoth creation. Waddling back to her cottage in the woods, she was met by the sound of creaking walks and deep lustfull moans. As she opened her bulging front door, she was met with a swollen fat teat leaking a sweet scented milk like a wide open faucet.
It certainly made life easier, being able to eat whatever you wanted without ever having to worry about your body changing, let alone fattening or growing. Though the idea to make all the weight that would have been gained be transfered to a...clone? Maybe it was an easier spell than conjuring something else, but for a witch as brilliant as her, she was clearly mind-foggy from all the delicious temptations she wanted to stuff herself with.
She could have made a portal to transfer added mass into, or maybe some magical machination that'd turn adipose into magic powder, who knows... The clone, in many ways, was actually a stronger version of the witch, even if less wise, and 'cursed' with constant weight gain. This curse made their mind change its opinion on gaining, and instead of sharing the opinion of wanting to be skinny, like the witch herself, her clone wanted the exact opposite - being as big, fat, blobby as possible. Though this required the clone to use magic too, and cast a gluttony spell on the witch that had created her...
Eating nonstop, to the point where her own initial spell was losing its power, making her clone grow to tremendous proportions, but also destroying her wishes of remaining skinny herself, as she rapidly went from chubby to utterly massive, hundreds upon hundreds of pounds in her belly alone, and she had a massive fat ass and gigantic tits to match.
Though as she waddled to confront her clone, not just about the gluttony spell but also about the fact she had no way to get herself more food, she discovered her clone was beyond immobile, filling the entire cottage with her body, and...lactating. A slightly purple tinted, glowing milk that was clearly magical in nature. The clone didn't notice the witch's arrival until the witch started fulfilling her gluttony curse, and drinking the milk straight from the source, laying her fat body on top of the much bigger clone and sucking on her tits.
The cottage didn't hold on for long, and was completely destroyed by the clone's growing body. Soon enough, the rest of the forest would be facing the same smothering destiny while the witch started matching the size she discovered her clone at, before latching (and never releasing) onto one of her boobs.