Starting with the '72 Oceanus models and excepting the '73 and '74, that "outline" is found in most of the Standard paint schemes and does frame the tank nicely to my eye.
You got me looking closer. In the above pic, I marked with a dotted pencil line the "break" in the metal of the tank on an original paint XS2. Except for the very rear, the stripe scheme ignores, and in fact obliterates the break. The XS2 scheme always looked "off" to me, and I think this is why. Also, the stripe comes too close to the badge.
The XS1B faithfully follows the "break" in the tank. And, there is good clearance around the badge. But, it needs that bottom stripe I added to ground it.
The Oceania models follow the break, except at the rear...
Here's what I mean. The rear of the brake is shown with dotted line. Why did the designer do this...
...because the black stripe would have become one with the black seat, and that would have been too much black, and too heavy. This is sleeker.
The clearance around the badge is good, but the bottom black stripe is too wide. It's too heavy, and subtracts from the flash of the orange.
Also, note how the chrome trim strip on the seat, does not visually line up with the bottom of the tank -- because the bottom of the tank disappears in blackness...
...They do line up on the XS2...
And they visually line up on the XS1b (and XS1) because the bottom of the tank has color, not blackness.
Says the Fine Arts grad.