Of course you do! If your body is tens of pounds lighter, then you don't need the extra muscle to lug it around. This paper is about reduction in heart muscle, and of course your heart doesn't need to be as strong because there's less blood to pump and less tissue to fuel.
When you gain weight, you also increase the muscles needed to carry that weight around. If you see someone obese at the gym doing the leg press, you may be astonished at how strong their legs are. When you lose weight, you don't need that muscle anymore.
Our bodies are really good at providing exactly the amount of muscle we need for our daily activities (provided we eat properly, i.e. sufficient protein), so it's entirely natural that our muscles decrease as we lose weight, the same way they increased when we gain weight. Muscles are expensive to keep around when we don't need them.
Obviously, if you exercise, then you'll keep the muscles you need for exercising.
But this notion that weight loss can somehow be a negative because you'll lose muscle too, I don't know where it came from. Yes you can lose muscle, but you never would have had that muscle in the first place if you hadn't been overweight -- so it's not something to worry about.
