The body is an integrated unit comprising muscle, bone, connective tissue, cavities, and organs filling the cavities. Oh yeah, there's the brain, too...I always forget mine.

The point is that due to our encapsulating skin, we're built sort of like a sausage....squeezed in between a layer that makes us take a shape. When we insert a thing, even a thin pacemaker, it will displace some tissue, AND....AND..it displaces or disrupts connective tissue. While you may get away without any puckering or whatever from displacing or disturbing connective tissue (and I wouldn't bet on it), inserting the pacemaker or ICD behind muscle must necessarily displace some of that muscle. If the muscle can't go sideways, or back toward the ribs (not likely, right?), and if the device is slid under/behind muscles on the side and front of the upper chest, it stands to reason that it will RAISE that tissue outward, toward the skin. And that is what we feel, and in certain angles of light, see.

Could be worse....I get lipomas every ten or fifteen years. Lumps of fat deposition, benign, that look unsightly. Very common, but I look goofy in a bathing suit.