You know that chromosomes are made of long molecules of DNA, scattered along which are stretches called genes. Genes are essentially recipes for the proteins that make up organisms and make organisms operate. At particular times in particular cells, genes "express" -- meaning they dump their information into a messenger molecule that take it to other parts of the cell that follow the recipe and create a protein.
Just exactly when genes express is just as important as what protein the gene is a recipe for. When genes express controls how organisms develop and operate. It affects when your bones grow and when you reach puberty; when you sleep and wake; when you get sick or heal; when you grow hair, gain weight, get a sunburn, get mad, or get pregnant.
It has everything to do with how many adipose (fat) cells and how fast your metabolism burns calories -- in other words, how you put on weight.
Part of the rule book that each gene follows to know whern to express is built right into the gene -- it's called the "promoter region" (or sometimes a "promoter gene" or just "promoter").
But some important non-DNA compounds also affect gene expression. Some become attached to the DNA and have major effects on gene expression without ever changing the DNA itself. The methyl groups and histones in the graphic are examples. These "epigenetic factors" can play a key role in silencing or activating genes. They are "programmed" throughout your life, with particularly important programming happening early on -- like in utero and during childhood.
There is evidence that many chemicals, especially agricultural pesticides, alter epigenetic regulation in ways that increase obesity (such compounds are called obesogens"). A good recent example: the fungicide Triflumizole was found to increase expression of obesity-causing genes in mice.