I’m the type of person that likes Obesity-type of magics to carry some sort of drawback, so Option B would probably describe me the best. Whether that side effect is a desired outcome or not can vary depending on the type of magic used.
Here’s a little food for thought, based on some of the games I’ve played and the ideas I’ve come up with for my own rules over the years.
Since a lot of magic in various game systems use blood, or the essence of blood for their power, an obese mage would have more blood in their body and potentially be more powerful, In Pathfinder 1e, there is a class called Bloodmage/Bloatmage that operates on this premise, although they emphasize the blood aspect more than the obesity aspect.
Essentially, instead of flavoring spells with “calories”, magic spells are powered with a person’s blood or essence much like normal, but an obese person is a bigger container, and thus can be more powerful the fatter they are, at the expense of all of the drawbacks of being obese.
In other game systems, magic puts a toll on a person’s body: physically, mentally, or both. Pyromancer from the Warhammer universe often dance a fine line between their love of fire, and the toll it takes on their body as they constantly push themselves fight the allure of their magic. In the end, they oftentimes become either withered people, or they finally loose control entirely that they go up in a nova of flames.
It wouldn’t be too much of a stretch to flavor something in more obesity-related theme instead. For instance, an order of Lipomancers could have ties to some sort of entity of gluttony, obesity, hedonism or greed; , or something more benign such as fertility, harvest, or wealth. This entity empowers their followers with magic, but a side effect of using their magic is that you gain a little weight every time you tap into their powers.
It could be an unwanted side effect - the powers of a God of Gluttony are powerful, but he leaves his mark on you each time you call on his power by making you a little fatter and become a mark of shame, or a desired effect, such as power from a god of wealth (fat people in medieval eras were associated with wealth and beauty). However it goes, the mage, as she gets fatter and fatter would potentially be more powerful, but is more and more hindered by her body. She might be able to use magic to fortify her body, but that too could cause her to gain weight faster. Eventually she might reach a point where she is too big to adventure (too fat to move, or too fat to fit through dungeon corridors), and be forced to retire.