Fat Tax or Weight Tax

Globally, there are more than 1 billion overweight adults, at least 300 million of them obese. Obesity and overweight pose a major risk for chronic diseases, including type 2 diabetes, cardiovascular disease, hypertension and stroke, and certain forms of cancer. The key causes are increased consumption of energy-dense foods high in saturated fats and sugars, and reduced physical activity.
WHO

"More than 25,000 people die of starvation every day, and more than 800 million people are chronically undernourished. On average, a child dies every five seconds from starvation."
WIKIPEDIA

Can you find any useful connection between them?

If these overweight (Obese) people can make some changes in their eating behavior, most of the death from starvation can be prevented.

And there is one way to do so proposed:

Imposing Fat Tax:

Popularly known as the "fat tax" or the "Twinkie tax," the concept first gained widespread attention in 1994. Foods with a high fat or sugar content would attract an extra tax.

But a fat tax would hit lower income families who already spend a higher proportion of their income on food and drink. Any tax on food could be seen as “regressive”, i.e. having a greater relative effect on the poor.

So why shouldn't the governments impose tax on the basis of weight (BMI, Body Mass Index)?

There is a tiny percentage of people with genuine metabolic problems that cause them to pile on the pounds. They, of course, deserve sympathy rather than contempt.