Density is about the interior too, not just the surface, so it's a mistake to say that something "looks dense". Even with relatively simple materials like metal or wood or stone, you can't use your eyes to judge their density.
for one, we don't just see the surface, but we see whats underneath the surface as well. for example, we can see blue veins. we can see muscle striations.
for two, we can make associations between what something looks like and what its interior is like. for example if we see a piece of foam we know from experience that the foam is not very dense. and if we see a piece of lead we know that it is very dense.
when looking at a muscle, if we have prior experience feeling, flexing, observing muscles of different types (shape, size, conditioning, density, etc), then we can make associations between what the muscle looks like and what its interior characteristics are.
for example when you look at the muscles on phil heath they appear like balloons, sort of hollow on the inside. this is not just because of the smooth skin, but also because of the amount of types of striations, the actual thickness of the muscle, th shape of the muscle, etc. when we look at johnnie jacksons back it appears extremely dense, because it looks very hard like concrete, you can see thick bundles of fiber, you know if you've been around muscles before that johynnie has a very dense back.
