Hiya OP! I’ve never seen fat shaming here, but you’re right that it is a problem within occult communities. People have this flawed idea that by following scientific health standards and guidelines, they’ll be able to alter their bodies (the American Heart Association (or one such American association of cardiologists) actually says bacon wrapped shrimp is healthy for people with high blood pressure! Watch What the Health). Corporations fund these so-called scientific studies, and use that cash to obfuscate results that would injure profits (hormone and medicine treated animal products are cigarette pack-class health dangers). This is true for any industry that could be damaged or supported by science- they give the scientists the money to do research, but only when their results match corporate agendas. Diet science, food science, and more are all heavily saturated with studies that just aren’t real. There’s multiple documentaries about this, including one where an amateur sent a rewritten Mein Kampf in the form of a study, and it was accepted into a major journal that is now suing the author.
My point is that everybody has a different body. The adage of ‘eat less, move more’ only even works when you don’t have psychological programming or physical diseases that impact the rate at which you gain or burn weight. Some people are able to eat processed sweets year round and not really have any bodily effects, while some are so allergic to nuts that a compound used in food that hits the air they breathe can trigger a fatal reaction. It’s never just black and white, there’s a whole spectrum of variables that need to be addressed before you can come close to finding common ground in operant human bodies. Here’s a rabbit hole: there’s no such thing as ‘junk DNA’! 
The issue is that occultists actually believe the all-too-real junk science that’s peddled by corporations behind the scenes. So, they assume athleticism = magical power, when really the threshold for an athletic body differs with each actual body. Fat is considered a blockage or inhibitor to achieving spiritual understanding, but, the same minds spouting that drink flouride and lead-laden waters. Some people do choose to be fat, consciously or not, but the problem there is the societal layers that have programmed them that way, including a lack of access to healthy food due to its overpricing.
Some people are consciously fat, some were made to be fat, and some were given no choice in the matter at any level. Psychological programming and societal factors are more to blame in these respects, where the person isn’t diseased and can’t be anything other than fat.
IMO, OP, the people who say that stuff don’t understand how bodies (or magic) works. The notion to address it psychologically first hits the board, but it doesn’t work without addressing your subconscious and unconscious layers. Disease aside, IME the most ingenious solution to fat is to enchant it to generate and retain heat, along with focused fire breathing to those ends. Dead fat cells are removed with exhalation in minuscule amounts, so, just kill them if they’re bothering you!
Contrary to popular belief, there is wiggle room when it comes to legwork for magically augmenting the body. Putting your mind through the sensory rigors of a gym is a few percentages of effectiveness off from actually going to and using one IIRC. IME, weight is just a number, and if you weaponize your mind to control it you get a lot of variables. I once gained thirty pounds, dropped eighty, and gained another twenty over the course of a day and a half! The magic you do can influence it, sometimes drastically like what I mentioned, but the mind is the owner of the body, so if you take total control of your mind, you can learn how to do the same with your body.
