The relationship is more biochemistry = more opportunities for cancer. Obesity causes increases in a broad range of hormones as well as a generally higher rate of inflammation and stress, which can impact gene expression and cause higher rates of mutation, leading to higher cancer rates.

https://www.cancer.gov/about-cancer/causes-prevention/risk/o...

This connection is seemingly true for tall people[1]

1. https://news.ucr.edu/articles/2018/10/26/study-explains-why-...

There is a theory that that has to do with endocrine disruptors, bisphenol A & suchlike. A related theory links the increase of hyperthyroidism in cats with the increasing popularity of canned cat food, it's chemicals from the can liner interfering with the thyroid.

There is a correlation between obesity and poor gut microbiome, which could be an explanation for some colon cancers. (There are a lot of caveats and uncertainty, but this is an active field of study).

Obese people probably eat terribly too. Candy, fast food, snacks, soda etc. No one is getting obese eating broccoli and chicken breasts