On Science Abuse they do an experiment every week called "Fat V Thin" where they get an extremely thin and fat man and put them in extreme conditions- ie- a very cold room, and measure their temperature. It is 50-50 who comes off as the better survivor.
Don't forget that in olden days being fat was seen as something to aspire to as it signified wealth. My Mum has always been chubby- not fat- but rounded, and I know that this will stand her in good stead health wise. Osteoperosis etc. She has not had a day's illness in her life. Babies are born fat because this way they stand a better chance of survival. Nature is telling us here that fat is good. Seriously- look at a baby! Rolls and rolls of fat on every limb etc!
Fatness is a problem depending on cause. By eating pies and drinking too much beer, you are becoming unhealthy, and the fat is a by-product. Fat on the outside also eventually means fat on the inside. The body tries to store fat away from vital organs, like on bums, hips, thighs in women. In men it seems to go on stomach which is a bit more dangerous as it is near to vital organs. Eventually the inside of the body has to store fat too, and interferes with the working of the organs- heart etc. It also slows you down physically and makes the body have to work harder.
I have had two of my very thin friends die recently (62, 58), and another (51) just diagnosed with Parkinsons. The latter was always a food fascist, and forced his kids to be vegetarian, and made them walk home from school (miles) in the rain as it would keep them "nice and thin". Well, being thin has not stopped him from getting ill at a young age. The first two friends both died of brain tumours, and were very slim all their lives.
As always, I guess moderation is the key- avoid extremes. Being thin does not guarantee you a longer or happier life, but if you are too fat to walk and drink heavily or eat more than your body needs, then this will