Booze is partly to blame. Women drinking pints. Getting shittered, eating a kebab on the way home. A general lack of fucking class.
It's odd that here in Thailand, women tend to be stick thin yet their diets are full of sugar. When my wife gets a drink for herself - like a Thai iced Tea, I can't drink it at all. It is nothing but sugar. Tea, sweet condensed milk and 6 or 7 spoons of sugar on top...

Then she'll go to 7-11 and get a litre of Red Fanta and guzzle it in front of the TV.
There's also a ton of deep fried food she eats too. Most breakfasts it's deep fried fish or pork, yet she remains stick thin. If she hits 42kg, she starts dieting. There's me with 6 eggs and cottage cheese and I have to run 5k a day just to stop expanding.
Obviously diet is also shitty in the UK but a different kind of shitty. In Thailand is they don't have a tendency to store food for long. You buy food fresh every day and so very little of it is processed food. In the UK, you nuke stuff, buy it from fast food joints, it's barely food any more.
I remember having here lunch with one English woman who was at least 250lb and I offered her some rice and she said "no, it makes you fat". Yup - all them fat Asians should take a leaf out of your book...

Anyway - I think poor genetics plays a role more than lack of willpower. I see plenty of women with no willpower that are completely getting away with it.
Still - if they are happy in their own skin. Why not? It's not like you have to fuck the fat ones is it?