Your bums look smaller in black trousers
Looking sexy and gorgeous is a matter of knowing your body, knowing what to wear and how to wear right kind of clothes. Dressing up is an art and if one can master this art then one can be on top of the world. The important thing about dressing right is you should look slim and tall in whatever you wear, no matter what your weight is. Even fat women can look slim if they know the tricks of fashion.
Your bum does look smaller when you dress in black, at least if viewed from behind. The reason is that we can perceive a shape only if what we see appears in different shades or colors. If you wore white trousers the shape of your behind could be inferred from the slight shadows cast by its contour. In black clothing, the shadows are invisible and the shape appears flat.
The claim is true and it is because of the uniform dark color of the clothing. Our perception of the shapes of surfaces, among other things, depends on minute shadows and patterns on the surface. See how much easier it is to notice wrinkles on a light shirt than on a dark shirt.
A similar illusion occurs with striped clothing, but it depends on whether the stripes are horizontal or vertical. Horizontal stripes give the illusion of widening while vertical stripes make a person taller and more compact. So a larger person wearing vertical stripes will take on a more flattering shape. But if a person wears horizontal stripes, their proportions will be accentuated sideways.