Middle age spread

I used to feel ok about myself apart from the usual paranoia about minor imperfections. Now I'm getting fat. Probably due to stopping smoking (well done me) and being on beta blockers. My metabolism has slowed to a sluggish pace and the weight is piling on. Scarey. It's got to go. Where does all that spare weight actually go to? How come I still easily fit into size 10 M&S skirts? Have I misread the label? Does it actually read size 10 for fat birds? It's a mystery.