another thing to add is almost universally, the reasons pets become unhealthily overweight is because they Do Not Know When To Stop Eating. they are animals, they will often try to eat as much as they can bc their survival instincts tell them they dont know when their next meal will be. they do not have body autonomy, we entirely control what and when they eat.
like for example its Very obvious in “simpler” animals like reptiles. once my snake finishes eating one large mouse, she is immediately looking for more bc the scent is on my hands. were i to give her another mouse, she would eat it, and continue to do so. some snakes will stuff themselves to the point of their stomachs rupturing, when enabled to. this behavior is fairly universal, and many owners will overfeed their snakes to the point of harm because they are “still hungry”.
in the wild this trait is useful bc they must be constantly active and may go months without a meal, but in captivity allowing this to happen is mistreatment and would lead to a snake being very unhealthy.
aside from the basic fact that fat people deserve respect regardless of whether they are “healthy” or not and medical ableism figures HEAVILY into what is considered “unhealthy”, overweight people Can be objectively healthy. this is not the case in every other species bc other species of animals bodies react to being overweight in different ways than human beings. like in dogs alone there are Dozens of health complications related to being overweight that do not happen to, or cant even be applied to human beings.
people are fat for different reasons, its an extremely complex and multifaceted thing bc we Have body autonomy and we are Not just animals living on pure instinct. our body politics are based in our physical nature as the human species, our culture, its institutions, etc. it is very unfair, and can even be unintentionally dehumanizing to apply the logic of one to the other.