steelheads are rainbow trouts that go into the larger bodies of water to grow big, and return to spawn. when they live in the larger lakes or ocean they turn silver with a black back, hence steelhead.
when they return to spawn, initially they look like a steelhead, as the time goes by, they turn more and more like a rainbow trout coloration.
rainbows do grow that big, but usually in rivers, and are rarer, and do not grow that big and never stocked in smaller creeks at that size. you'll see large adult rainbows stocked in smaller lakes more often.
this also applies to salmons, and all different trouts as well. when they go to the lake most turn silver, and change back to original color and shape when in the rivers.
they develope different mouth pattern, called a kype, in the males, they develop a pronounced hooked jaw, as you can see in your fish, its more and more obvious as time goes by.
most salmons die after spawning, and many trouts and steelheads die also, but not all, and a lot of steelheads actually survive to the lakes again and return to spawn again! hope that helps clear up the confusion!