Contrary to what social justice warriors and fat-apologists want us to believe, it is a well-known scientific fact that being obese is very bad for your health. 

Obesity, whether measured by BF%, BMI or simple waist circumference is well known to be a predictor of hypertension, diabetes, pre-diabetes and cardiovascular ill-health as well as other metabolic health-issues.  This holds true across different countries, regions and for all races and socio-economic groups.

Unfortunately, these health complications caused by obesity correlate almost exactly with the biggest risk-factors for COVID-19.  In pretty much all the published data so far hypertension is the worst cofactor showing up in 15-30% of COVID-19 cases, cardiovascular disease has been a factor in 10-50% of COVID-19 cases and diabetes is present in between 9 and 30% of cases.  No data has been published regarding the co-incidence of pre-diabetes or insulin resistance, presumably because these are not normally considered a disease by the medical community (no matter how much we think they should be).

Overall, it is estimated that up to 75% of deaths from COVID-19 could have been avoided if the comorbid conditions were not present.

There are a couple of potential explanations for the greatly increased risk to COVID-19 patients when they also have one or more of the comorbidities.

The first is that all of these conditions increase or are caused by systemic inflammation, and actual deaths from COVID-19 are mostly caused by inflammation in the lungs or liver rather than by the direct effects of the virus.  This is known as Cytokine Storm and is a major factor in many other lung diseases.

A second possibility is that ACE-2, an enzyme that is expressed in the lungs and other tissues is elevated in many of these diseases.  ACE-2 is used by the SARS-CoV-2 virus as an entryway into the body, so elevated levels may make people more susceptible to the virus.  ACE-2 is often elevated in the elderly who are also at risk even without other serious comorbidities.  This enzyme elevation also applies to smokers who are of course more susceptible for other reasons too, including a weakened immune system and deadened lung protection function.