The truth is, there is no one way to be healthy. You can literally be a marathon runner and drop dead of a heart attack. Happens regularly.
If YOUR health matters TO YOU, then do some experimenting and figure out how YOUR body works best. What makes YOU feel good in YOUR body? Do some research on and then talk to your doctor about something called “metabolic health” – basically the idea that if your heart rate, blood pressure, A1C, cholesterol, and other “numbers” are good, then you’re “metabolically healthy”. But remember that people who have “good” numbers can have other health problems, too, and you can have some “bad” numbers and still be pretty healthy. Do some research on “Health at every size”, too. Consider the idea of “health” in the context of chronic illness, mental illness, and disability, too. Are you “healthy” if you have chronic migraines, depression, or scoliosis, but still do well by other measures? How about asthma? Or diabetes, in fact?
There are lots of ways to be healthy or unhealthy. It’s not a straightforward thing, or a single thing.
And it sure as shit doesn’t show from the outside. You absolutely cannot look at another human being and know that they are healthy or unhealthy.
Nor is the health of any other human being in the entire world your business unless they invite you to make it their business. Yes, even your family members. (The exception is your own minor children. Adults who have become unable to take care of themselves and for whose health you are responsible have, presumably, asked you or authorized you to make their health your business.)
Do not assume that there is ANY weight that is “too thin” or “too fat” to be healthy. Find a weight that works for you, both in that you can maintain it without doing things that will negatively impact your metabolic health (like dieting or overexercising, neither of which is good for you and neither of which works to keep your weight down long term, and both of which can actually CAUSE WEIGHT GAIN long term, which leads to yo-yo dieting or weight cycling, which is very bad for your metabolic health long term) OR your mental and emotional health (by, say, causing you lots of anxiety), AND that feels healthy. Because when your body works the best it can for you, you can feel that.
And, again, don’t fucking worry about anybody’s body but yours. Don’t listen to people who try to make your health their business without your invitation, and don’t listen to people who tell you that your body ought to conform to some standard they have in their heads.
And finally, you have no responsibility or duty to anyone to be healthy, and you are not worth more or less as a human being based on your health. You are not bad if you are unhealthy or good if you are healthy. You’re a person, and that’s enough.
TLDR: Health is not one thing. There are many dimensions to and kinds of health. You can be healthy at whatever size you are (and that’s good, because there’s not a lot you can do to change your size in the long term, at least not to get smaller). Don’t worry about anybody’s health but your own. You are not bad if you aren’t healthy.