The word vegan often gets wrapped up in diet culture because when people think of the word vegan they think of not eating animals, GIVING UP pepperoni and cheese on pizza, McDonald's chicken sandwiches, milkshakes and grilled cheese.
This isn't wrong but not eating animals is only one aspect of veganism and to a vegan not eating animals isn't because of body weight or size it has nothing to do with human health. It has everything to do with not exploiting, consuming, harming and ending systems of oppression for humans and animals.
Veganism is a PHILOSOPHY, a way of life that has to do with much more than CHOOSING not to eat animals.
A plant-based diet on the other hand can definitely be marketed as a diet and is often only concerned about the health aspects of REDUCING consumption of animals. I hear it all the time "maybe I'll try going plant-based to lose weight". Unfortunately, some vegan messaging can also get caught up in diet culture because plant-based can also mean vegan and the health aspect of not eating animals is a big seller for humans.
Vegans can be fat. Vegans can be disabled. Vegans can get sick. Vegans can get cancer and have heart attacks. Being vegan does not make someone immune to any health issue.
Veganism is about animals, oppression and social justice. Not body size, look or ability.
Reducing animal consumption is great for health, environment and for animals.
The WHY behind the decision will be the reason it is a diet or a philosophy.
Many people go vegan for diet culture reasons, in my opinion that's why many people don't stay vegan.
It's the difference between "I can't have that pizza because it has pepperoni" and "I choose not to eat pizza with cheese and pepperoni because I don't agree with animals having to die for me to eat pizza, I don't agree with marginalized humans having to slaughter animals for me to eat pizza, I don't agree with mass amounts of methane and CO2 going into the atmosphere for me to eat pizza."
It's not a restriction to not eat animals when you are coming from a place of social justice, not diet culture.