Would an example help?
I know someone who has porridge for breakfast, a sandwich at lunch, and a relatively healthy evening meal. Rarely has fried food, doesn’t drink, about the only vice is the occasional chocolate bar.
She’s reasonably active; walks, climbs, rides a MTB when practical, and recently did the Leeds-Liverpool canal end to end.
She’s 5’6″ and 18 stone, which is medically “obese.” She’s desperate to lose weight, seen a Dietitian (who ostensibly said ‘carry on as you are, that’s great’), but simply can’t shift the weight.
The reason she became overweight is partly self-inflicted yes, and also partly down to external forces, but mostly because of an illness. This illness promotes fat storage, and being fat makes the illness worse. Catch 22.
So if you’re sitting there all clever going “fat people eh, why don’t they just eat less / get off their arses, and aren’t they funny” I suggest you count to ten before you reach for the keyboard again.