In the online feminist world, there's a concept dubbed "mansplaining". It's where a woman will share something which happened to her, and someone else (canonically a man) will "explain" the experience for her. For example, if dozens of men scarily harass a woman on the street, and she complains about it, someone pops up and "explains" that it wasn't scary or harassment, it was just people being friendly! Mansplaining, or "X-splaining" to keep it gender neutral, is not the passing on of information, it is the condescending denial of personal experience.
Recently there was a kerfuffle (or possibly a fracas) about "fat shaming". A non-fat comedian went on a unfunny rant about fat people and brought up a bunch of really dumb stereotypes about how terrible fat people are, and it's all their fault for simply eating too much. (As a bonus, she claims to be trying to help fat people lose weight by shaming them, while simultaneously claiming fat shaming does not exist.)
I'm not going to link to the video because fuck her, but she's hardly unique in her views. A whole lot of people think fat people are simply lazy and gluttonous, and that all they need to do is eat less. And yeah, scientifically, if fat people ate less, they'd lose weight! Case closed!
One, it's really none of your business, and no one owes you an explanation. Two, it seems that weight loss is simple, but not easy. As it turns out, once you are overweight by enough for a long enough time, losing that weight and keeping it off is one of the hardest things a person can do.
Since everyone loves personal anecdotes, I've lost 50 pounds! Three times! Oh…
And now I'm back to near the top of my weight. I lost weight last time by following a "common sense"-type plan. It took a lot of daily effort, for years. I avoided some foods and sought out others. On a daily basis I struggled. I lost weight. Then, life got a little more stressful due to a move and an upcoming baby, so eating the right amounts of the right foods was a little bit harder, and the weight started coming back.
It is entirely true that I am responsible for the amount and kinds of food I ate, and that it's "simple" to do it right, but it's not easy. The amount of "not easy" varies from person to person. Obviously genetics plays a role in how a person's body reacts to calorie reduction and diet changes. There are strong hints that the types of bacteria populating a person's intestines plays a role as well.
But even if those don't have any real role, once a person is obese, it's very difficult to lose weight. Which beings us back to thinsplaining.
It's not thinsplaining if you inform people that obesity is unhealthy. It's not thinsplaining if you provide links to research documenting the best weight loss techniques.
Thinsplaining is when a thin person "explains" to a fat person how easy it is to lose weight.
Thinsplaining is someone who has never had to lose much weight telling fat people that it's easy.
Thinsplaining is someone with no experience in the amount of "willpower" needed to lose weight telling people it's simply a matter of willpower.
"Just don't eat so much!" says the thinsplainer. "Just stop eating sugar!" says the thinsplainer. "Just never eat any foods you like again!", says the thinsplainer. That's… actually not that helpful? It's like someone who has never taken a test saying "Oh the SATs are easy! I totally had a pop quiz the other day and did fine, so I know what I'm talking about."
"Hey heroin users: stop using heroin!" "Hey depressed people: cheer up!"
"Hey fat people: eat less!" Thanks. I hadn't thought of that. GFY.