thinking of a futuristic concept of a fat suit as in 'suit that actually makes you fat'
Jumpsuit that is full of little sensors and electrical stimulators and nanite factories.
It doesn't directly break conservation of mass to make you fat, but it has all kinds of other tricks.
Somewhat like a plant, but far more efficiently, it steadily uses the energy of light, body heat, and motion along with atmospheric CO2, nitrogen, water vapor, and your own sweat to make a steady stream of extra nutrients and calories for you even if you don't eat... and ensure that it can also power, stretch, and repair itself, making it that much harder to get rid of.
A bunch of targeted electrical signals and nanites do a variety of things on top of that
It convinces you that you're hungry, suppresses fullness, fosters eating addictions and arousal responses to food, fosters strong aversions to *not* eating, makes your appetite and stomach capacity expand at an unnatural speed and to an unnatural extent, helps you digest things faster and more efficiently, and makes your fat cells eager to divide and eager to work together with your thyroid to pump out chemicals that further ease your weight gain.
You become willing to eat practically anything, especially anything high-calorie. Being fattening alone makes things taste incredible to you, and you start craving butter, oil, heavy cream, etc.
It also enhances your strength, but in a way that leaves you heavily reliant on the suit. Besides powerful contractile supporting fibers in the suit itself, it also stimulates your muscles to work more efficiently...
But in a way where your muscles are steadily getting more and more reliant on the suit's stimulation and guidance, suddenly becoming sore and weak without its help.
In a setting of nanites, there's no escaping external situations with abundant food.
Even your home becomes set up with a network of little nanite foglets from the suit, crowding out and co-opting other systems already in place, and creating the illusion that they can move objects and feed you 'telekinetically'.
If it has to, it'll just extend its artificial nutrition synthesis protocols to its external nano-swarms too, and with them able to harvest far more than the suit alone, it can just feed you with nothing but air, water, and a mix of light and household electricity.
Soon enough, you're so huge that even the suit can't get you to walk, and so its nano-swarms just endlessly feed you as you sit there in an addicted haze of pain and pleasure, constantly feeling uncomfortably bloated, but immediately begging for it to continue if anyone ever tries to rescue you.
