What else explains the fact that some people can eat like pigs and not exercise and remain super skinny
Because they don't do that. Any properly conducted study of what they eat over an extended period will demonstrate that they don't actually eat very much and burn off what they do.
Channel 4 did a programme called the secrets of slim people. I think there was one woman they couldn't explain, but for all the others, the 'slim people who ate loads but didn't gain weight' just didn't eat very much when watched carefully for a few days.
Going the other way, the Secret Eaters programme, which appears to be available on youtube, showed that the people who claimed that they 'didn't eat much' or 'lived on salad' simply 'forgot' about a lot of what they ate, or ate enormous portions. I remember a woman who ate huge portions of healthy food, like a washing up bowl sized fruit salad. The woman who 'lived on salad but couldn't lose weight' didn't think to count the things she bought from Greggs more than once a day when she walked past during her job as a traffic warden on the high street.
I eat curry and croissants most days and have 32ins hips
So? Depending on what else you eat, that could still add up to not very much.
Two croissants is less than 500 calories
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What is 'curry'? that could be absolutely anything and vary from under 300 calories to over 2000 calories depending on portion size, ingredients, sides and accompaniments, but say you had this for 750 calories in addition to your 2 croissants, that's only 1250 calories or say 1500 if you have butter, jam, cheese, ham or whatever with the croissants.
So even if you have something else each day, say a sandwich, salad or some soup, for under 500 calories, chances are that diet won't cause you to gain weight unless you literally never move.
Many 'inexplicably slim' people also undertake more day to day activity like fidgeting, never sitting down, or walking to work or being active at work. So burn more calories than an office worker who goes to the gym or runs a few time a week.