Never seen a skinny hippo either - so I guess there may be something "genetic" as a cause for some people.
Most however, will be caused by a combination of too many calories and too little excercise - and the problem is only going to get worse, given all of the labour saving inventions nowadays, just think, it wasnt so long ago that you were scrubbing clothes by hand (replaced by washing machine), walking or cycling everywhere (replaced by mass car ownership), sweeping and scrubbing floors (replaced by vacum cleaners) - even walking through an airport doesnt have to be done nowadays, `cos of those bloody "travelators".
Add to this the constant pressure to work harder/longer (sat at a desk/flight deck) - and a lot of people either dont have the time, or a just too plain knackered to do any propper excercise, or to cook a decent meal - far easier to stick a pile of processed crap in the oven for 20 minutes - which will contain high calories in quite small portions.
My other half has just been advised to give up sugar totally - you have no idea what sort of stuff contains sugar until you start reading the packets carefully - and we basically have to now cook everything from scratch - which is way more expensive (tastes great though).
Out of interest, whats to stop aircraft makers building aircraft a little wider, giving everybody an extra couple of inches in seat width, efverybody I know would gladly pay a little more (£10 shorthaul - £20-£30 long haul) for the extra comfort.