The ideal weight is a consuming preoccupation. Society aside, there is cast iron logic that there is an optimal skin-friendly approach to an ideal weight. Collagen in the dermis layer of the skin is responsible for the skin's vibrant plumpness. It also has a related fiber called elastin responsible for the skin's elasticity.
Rapid weight loss or gain erodes the elasticity of the elastin fibers in the skin. This is the most important reason that systematic weight loss is the skin-friendliest way to manage weight. Weight gain stretches skin - hello stretch marks - and weight loss causes skin to get lax. This loss of elasticity that doesn't let skin snap back is not worth the hassle especially if your genetically determined fat loss pattern ever involves losing fat on your face.
Dr. Gerald Imber notes in his latest book Absolute Beauty (an update of the Youth Corridor) that slowly losing weight allows slowly reacting skin an anatomical advantage of shrinking to fit underlying structures closely and attractively and prevent it from looking loose, empty and haggard even when you have gone through the fortitude of having lost weight.
So weight loss of anything more than a few pounds should be at the rate of half a pound a week and while it's normal to lose 2-3 pounds in the first week, this is because of water weight and it's just best to be moderate and maintain in either direction. To this end it's also best to weigh yourself every day if you are smaller since a pound on a petite physique just means more if its following a bingey holiday like Easter.
And in terms of social norms of slenderness - there is such a thing as too thin -normal amounts of subcutaneous fat goes a long way to help skin look and feel healthy. Finding optimal weight is one you have to determine a balance of energy and appearance and defend against the allure of bingeing and the harsh austerity of rapid weight loss.