That’s simply not true.
Although there is a correlation between serious Covid illness and being overweight, nowhere near 80% of Covid deaths are amongst the obese (BMI over 30). According to a study in the Lancet, it’s more like 30% who are obese and another 30% who are slightly overweight.
On the other hand well over 90% of those with serious illness or who die from it are unvaxxed so there’s really no comparison between the two groups.
A vaxxed fatty is much more likely to have a better outcome than an unvaxxed thin person if they both catch it.