Gram for gram tuna have more muscle than any other vertebrate which is one reason why fish are so good eat. But more importantly the muscles are layered rather than bundled and this is what makes the difference.

Each segment of muscles is called a myotome, and is separated from its neighbor by a sheet of connective tissue. These muscles are separated with collagen sheaths surrounding the muscle fibers from end to end. But these fibers do not run parallel the length of the fish, and instead each individual muscle is folded into the shape of a W. This is what gives the flesh its zigzagging effect seen below, and this geometry allows muscle contractions and the required motion to propel a fish efficiently.