I think the biggest from of all is the food and drinks that we eat and the amount of it we eat.
If you go into any supermarket, the majority of the food and drink for sale is sugary, fattening, processed food. The aisles are filled with jars, sweets, cake, crisps, ice cream, pop, alcohol, ready meals. There is less of the fruit, vegetable and fresh ingredients. The junk food is fairly cheap too.
There are more and more takeaways popping up everywhere, with deliveroo etc it's so easily available. We've had 3 new KFCs pop up and a new McDonald's all within a 2 mile radius. The local high street is full of pizza and kebab shops.
We've all had the wrong type of foods pushed onto us more and more.
Exercise is of course a factor but the biggest cause of obesity is eating too much.