The anthias is a good example. I was thinking of long tailed fish, where it could make a drastic difference. Not a reef example... but consider veil tailed mollys or swordfish. The tail can be as long as the body!

I have a bellus angel, also with a forked tail. The skinny little tendrils on her tail, if I include them, make her a 6 or 7 inch fish. I can't tell for sure, they're always dangling about in the current. (Justin's point comes in handy there.) If I measure just her body, she's only about 3.5 inches. That's quite a difference.